Method and apparatus for economical drift compensation in high resolution difference measurements and exemplary low cost, high resolution differential digital thermometer

ABSTRACT

A system for measuring differences in a physical variable, such as temperature, by utilizing predictable behavior in the relative time drift of offset curves for various circuit elements, including two sensors connected to a difference signal amplifier, an ambient condition amplifier, and an analog to digital converter. In an initial calibration mode, the system records several offset curves, stored in memory, correlating ambient condition measurements to offset measurements acquired from the ambient condition amplifier and the difference signal amplifier. Offset curves recorded in the initial calibration mode, correlating ambient condition measurements to measurements from the difference signal amplifier, include one curve recorded with both inputs of the difference signal amplifier held at equal potential and another curve recorded with both sensors held at the same value of the physical variable, over a given ambient condition range. Another offset curve correlates ambient condition to measurements from the ambient condition amplifier, with inputs to the ambient condition amplifier connected to voltages from a substantially time stable reference resistance bridge. These offset curves representing drift behavior, among electrical components, can be updated for time drift, at a single, current arbitrary ambient temperature, the measurements for which can be obtained quickly and applied as a time drift correction to the offset curves, without interrupting normal system operation, to provide a compensated difference measurement between the different values of the physical variable measured by the respective sensors. Additionally, the system dynamically tracks cumulative system errors, in order to calculate optimal system resolution, based upon current operating conditions.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This is a continuation-in-part of U.S. patent application SC/Ser. No.08/997,901, filed on Dec. 24, 1997 now abandoned, entitled “METHOD ANDAPPARATUS FOR ECONOMICAL DRIFT COMPENSATION IN HIGH RESOLUTIONDIFFERENCE MEASUREMENTS AND EXEMPLARY LOW COST, HIGH RESOLUTIONDIFFERENTIAL DIGITAL THERMOMETER.”

TECHNICAL FIELD

The present invention relates to measuring and recording devices andtechniques for compensating electronic difference measurement systemsfor the effects of electronic component drift over time and temperature.More particularly, in one exemplary embodiment, the present inventionrelates to temperature measuring and recording devices and techniqueswhich perform high resolution temperature difference measurements, onthe order of micro-degrees centigrade.

The present invention accurately resolves extremely small differences inelectrical signals, in a very low cost, highly portable apparatus thatcan be battery operated. In an exemplary embodiment, the method andapparatus of the present invention are directed to the measurement oftemperature differences, on the order of micro-degrees centigrade, byutilizing predictable behavior in the relative time drift of thermaloffset curves, for various circuit elements, including a differencesignal amplification means, an ambient temperature amplification means,and an analog to digital converter means. In an initial calibrationmode, preferably performed at the time of manufacture, the exemplaryembodiment records several thermal offset curves, stored in memory,which correlate ambient temperature measurements to offset measurementsacquired from the ambient temperature amplification means and thedifference signal amplification means, with both of said amplificationmeans connected to a measurement bridge, comprising two thermistors andtwo resistors, for measuring ambient temperature and temperaturedifferences (via nodes of the measurement bridge). Thermal offset curvesrecorded in the initial calibration mode, correlating ambienttemperature measurements to measurements from the difference signalamplification means, include one curve recorded with both inputs of thedifference signal amplification means held at equal potential andanother curve recorded with both thermistors of the measurement bridgeheld at the same temperature, over a given ambient temperature range.Another thermal offset curve, preferably recorded at the time ofmanufacture, correlates measured ambient temperature from the ambienttemperature amplification means to measurements from the ambienttemperature amplification means, with inputs to said ambient temperatureamplification means shorted together or, alternatively, connected to oneor more reference signals which in the exemplary embodiment arepreferably voltages from a reference resistance bridge, preferablycomprising substantially time stable (not necessarily temperaturestable) resistors. The method and apparatus of the present inventionrequire few components, and no precision active or passive components,resulting in low power consumption, and low cost. The present inventionovercomes time and temperature component drift, by utilizing the factthat the thermal offset curves, acquired in the initial calibration mode(preferably at the time of manufacture), drift with time in apredominantly linear fashion relative to one another. Consequently,during normal operation, these offset curves representing temperaturedrift behavior, among electrical components, can be updated for timedrift, at a single, current arbitrary ambient temperature, themeasurements for which can be obtained quickly and applied as a timedrift correction to thermal offset curves, without interrupting normalsystem operation. Additionally, the present invention dynamically trackscumulative system errors associated with the method of the presentinvention, in order to dynamically calculate optimal system resolution,based upon current operating conditions (rather than based upon moregeneral component drift specifications).

BACKGROUND OF THE INVENTION

Various electronic systems exist for measuring extremely smalldifferences in sensor measurements, such as temperature, for use inbiological and physical analysis. It is known in the art that active andpassive electronic components in such systems are subject to time andtemperature drift, and that under normal operating conditions, theamplitude of time and temperature component drift is typically muchgreater than the amplitude of other inaccuracies generated by systemcomponents, such as amplifier noise voltage, noise current, and resistornoise. Consequently, component time and temperature drift aresignificant limiting factors to high resolution measurements, such astemperature difference measurements. To address the problem of componentdrift in electronic measurement systems generally, various approaches tocompensate for drift have been devised.

For example, U.S. Pat. No. 5,253,532 (Kamens); U.S. Pat. No. 5,042,307(Kato); U.S. Pat. No. 4,611,163 (Madeley); and U.S. Pat. No. 3,831,042(La Claire) disclose electronic measurement systems (principallydirected to pressure sensing, in the preferred embodiments) whichinclude additional hardware components that change their electricalresistance, or other electrical parameters, with ambient temperature, insuch a way as to compensate for thermal drift in measurement systems towhich they are electrically connected. While such hardware compensationsystems provide some compensation for thermal drift inaccuracies, theydo not compensate for component drift over time, particularly the driftof sensors, such as thermistors. This would be sufficient to precludetemperature difference measurements, with resolution on the order ofmicro-degrees centigrade, if these techniques were applied to thatpurpose. Additionally, such hardware based compensation techniques donot readily compensate for component drift, resulting from the combinedtime drift characteristics of multiple system components, located atdifferent parts of the system, with different thermal driftcharacteristics, and subject to non-uniform aging. In any case, theability of the above hardware based compensation systems and techniquesto compensate for system thermal drift are limited by the extent towhich the particular technique tracks with thermal drift of the overallsystem, over time and temperature. Consequently, such techniques wouldnot provide sufficient compensation for component time and temperaturedrift to permit differential temperature measurements, with resolutionon the order of micro-degrees centigrade, if these techniques wereapplied to that purpose.

Other hardware compensation techniques, such as disclosed in U.S. Pat.No. 5,616,846 (Kwasnik); U.S. Pat. No. 5,171,091 ((ruger et al.); andU.S. Pat. No. 5,132,609 (Nguyen), require a time and temperature stablereference signal, and U.S. Pat. No. 5,351,010 (Leopold et al.) requiresthe use of precision analog amplification hardware and costly time andtemperature stable resistors. The required precision analog componentsin these systems results in increased cost, complexity, and powerconsumption. Moreover, these systems do not compensate for time drift ofpassive components, such as thermistors, which would be sufficient topreclude temperature difference measurements, with resolution on theorder of micro-degrees centigrade, if these systems were applied to thatpurpose.

Additionally, U.S. Pat. No. 5,162,725 (Hodson et al.); U.S. Pat. No.5,065,613 (Lehnert); U.S. Pat. No. 4,958,936 (Sakamoto et al.); and U.S.Pat. No. 4,464,725 (Briefer) describe electronic measurement systemswhich compensate for thermal drift, and other system inaccuracies, byutilizing a computer, and memory for storing known temperature behaviorof a measurement system, at various calibration temperatures. That is,system inaccuracies due to temperature drift are recorded at specificcalibration temperatures. This stored temperature behavior is then usedto interpolate system inaccuracies due to thermal component drift atoperational temperatures within the calibration range. This has beenaccomplished by using mathematical formulae to model thermal offsetcurves (e.g., using a parabolic interpolation, such as the LaGrangemethod, to plot offset curves, based upon discrete offset measurements,at discrete ambient temperatures), and then during normal operation,using a said formula, with a current ambient temperature measurement, todetermine expected circuit offsets for the current ambient temperature,so that actual system measurements during normal operation can beadjusted for the effects of said expected circuit offsets. The prior artmeasurement systems, which utilize a computer, can provide time andtemperature compensation based only upon the most recent referencecalibration data, the acquisition of which requires that the system becycled through an entire temperature range, and is sufficiently timeconsuming to prevent, or significantly interrupt, normal systemoperation. Other techniques that utilize software compensation, such asdisclosed in U.S. Pat. No. 4,532,601 (Lenderking et al.), as well as inU.S. Pat. No. 4,464,725 (Briefer, referred to above), require the use ofa time and temperature stable reference signal, which increases cost,complexity, and power consumption. U.S. Pat. No. 4,959,804 (Willing)utilizes time and temperature stable passive components, which, evenwith costly bulk metal foil, or wirewound, resistors, would not providethe accuracy necessary in temperature difference measurements, withresolution on the order of micro-degrees centigrade, if this techniquewere applied to that purpose. Such time and temperature stable bulkmetal foil resistors (e.g., manufactured by Vishay Electronics FoilResistors, of Malvern, Pa.) and wirewound resistors, such asmanufactured by Dale Electronics, of Norfolk, Nebr., are one to twoorders of magnitude more expensive than standard metal film resistors,which provide comparable time stability, but are not nearly astemperature stable. Furthermore, U.S. Pat. No. 4,959,804 (Willing,referred to above) updates a previously recorded temperature curveaccording to the two endpoints of the curve, thereby ignoring variationsthat might occur at intervening points along the curve, over time, aswell as time drift in temperature measuring thermistors, which driftsufficiently over time to invalidate temperature differencemeasurements, with resolution on the order of micro-degrees centigrade,if it were applied to the purpose of high resolution differentialtemperature measurements. U.S. Pat. No. 4,651,292 (Jeenicke) relies onupdating a point on a measurement curve, requiring, however, therestriction that the sensor curve characteristic be linear (not the casewith temperature sensors, such as thermistors, and not sufficiently so,to provide resolution on the order of micro-degrees centigrade, evenwith known thermistor linearization techniques) and that ambienttemperature measurements not drift with time, to the extent thatmeasurement accuracy would be affected, making this technique unsuitablefor a differential thermometer with resolution on the order ofmicro-degrees centigrade, if such a technique were to be applied to thatpurpose.

In the above approaches, as they would relate to a temperaturedifference measurement system, utilizing a pair of thermistors (e.g., ina thermistor-resistor bridge arrangement), it is relevant to note thatdifferences in thermistor resistance-temperature curve characteristics,between two thermistors, result in a difference in the two thermistorresistances, throughout an ambient temperature range, that variessignificantly with ambient temperature. For instance, YSI 460 series“Super-Stable Thermistors”, manufactured by YSI, Incorporated, of YellowSprings, Ohio, are characteristic of well matched, commerciallyavailable thermistors, and are matched to within 0.05° C. of each other,between 0° C. and 50° C. This means that within the 0° C. to 50° C.range of operating temperatures, the difference in thermistorresistances may change by as much as 0.001° C., relative to each other,for each ambient temperature change of 1° C., a significant amount inmeasurements intended to resolve temperature differences on the order ofmicro-degrees centigrade. The above approaches, as they would relate toa temperature difference measurement system, utilizing a pair ofthermistors, do not provide a means to compensate for this effect.Additionally, in order to minimize common mode amplifier error, the useof bipolar power to the measurement bridge is often preferred in theabove prior art, as are high precision amplifiers, which typicallyrequire bipolar power, resulting in added cost and complexity, comparedto a single-ended power supply architecture.

In U.S. Pat. No. 5,295,746 (Friauf et al.), directed specifically to thetechnical field of temperature difference measurement, with resolutionon the order of micro-degrees centigrade, it is pointed out that anumber of digital thermometers exist, which, however, have accuracylimitations on the order of one hundred milli-degrees centigrade. U.S.Pat. No. 5,295,746 addresses some of the limitations of the prior art,in this respect, by using a computer to maintain a thermistor-resistorbridge in a balanced state, to provide a means for adjusting thermistorpower dissipation and to null out thermally generated offsets in thesystem's analog to digital converter, digital to analog converters, andamplifiers, for a given thermistor power dissipation, for the currentambient temperature. However, this requires that the bridge circuit bebalanced with extreme accuracy, requiring the addition of two digital toanalog converters to the circuit, preferably employing high resolution,in order to achieve high resolution temperature difference measurements,resulting in added component count, cost, and power consumption.Additionally, no means is provided to compensate for time drift ofthermistors, which typically amounts to ten or more milli-degrees/year(e.g., YSI 44018, manufactured by YSI Incorporated, of ellow Springs,Ohio), a significant figure in temperature difference measurements,intended to approach micro-degree centigrade resolution. Additionally,software calibration of this system, for a current ambient temperature,is undertaken at the time when temperature difference measurements areundertaken, yet requires that bridge thermistors be completely powereddown first, so that there is zero voltage potential across the bridgeduring calibration. That is, system calibration followed by continuedoperation in the temperature difference measurement mode must beundertaken by first powering down the bridge thermistors, and thenpowering them up again. Due to self-heating properties of thermistors,when a voltage is placed across thermistors, time is required for thethermistors to reach equilibrium with ambient temperature, which they doasymptotically. In cases where temperature difference measurements onthe order of micro-degrees centigrade are to be resolved, this poweringdown and then powering up of the bridge, until the thermistors arewithin micro-degrees centigrade of equilibrium, adds significant time tothe calibration process. Since each calibration offset measurement isassociated with a specific ambient temperature, a calibrationmeasurement (and, therefore, an ambient temperature) must be associatedwith each temperature difference measurement. Calibration measurementsperformed before and after a measurement run can conceivably be used tointerpolate linear changes in ambient temperature with time, during ameasurement run, but this places an unrealistic limitation on ameasurement system which desirably operates under normal atmosphericconditions, in which ambient temperature changes may not be linear withtime. Therefore, a calibration measurement must be undertaken for eachtemperature difference measurement, in which ambient temperature may nothave undergone a linear change, thus adding significant time to themeasurement process. Similarly, U.S. Pat. No. 5,351,010 (Leopold et al.,also mentioned above) requires that current be reversed through zero, inresistive sensors, for each calibration, as well as requiring precisioncircuitry, that increases cost, complexity, and power consumption.

Additionally, the prior art does not provide a means to dynamicallyquantify compensation inaccuracies, resulting from the particular driftcompensation technique used. These inaccuracies, inherent in the abovecompensation techniques, may vary widely, depending on specificoperating conditions, such as: ambient temperature; number and locationof temperature compensation/sensing devices in the system; thermal andtime drift homogeneity among system components; time elapsed since areference calibration (where relevant); and system warm-up status. Inthe above prior art, a general specification based upon a combination ofindividual system component drift tolerances, taken as a whole, canconceivably be computed to account for temperature and time driftlimitations of a system. Based upon a given calibration, and/orcompensation technique, over an intended temperature span, such acomputation could be used to provide a limitation to achievableresolution in the above prior art, in a given operating environment, fora given calibration/compensation technique, over an expected temperaturerange of operation. However, in order to be reliable, such a computationwould need to take into consideration factors which include long termtime-drift characteristics of active and passive components, includingamplifiers, and mixed-mode devices, such as analog/digital converters,as well as time drift of passive components, such as resistors andthermistors. An additional source of error to take into consideration,in devices expected to perform to specification when they are turned on,includes time versus drift behavior during system warm-up. Consequently,although a general specification for the ability of a technique tocompensate for component time and temperature drift may be calculated,by combining manufacturer supplied drift specifications for relevantcomponents, the actual value of errors associated with uncompensatedcomponent drift, in a given compensation technique, may changesignificantly, depending on the above factors, so that rather than beingoptimized, based upon current operating conditions, such a calculatedspecification must be set high enough to anticipate worst caseconditions.

Such calculated resolution limits, for a given system, over a giventemperature span, are often used to estimate performance in bridgemeasurement systems. However, it would be highly advantageous indetecting extremely small variations, approaching the limitations ofmodern electronics, to dynamically use limitation information thatimproves upon such absolute estimates, whenever possible. For example,system limitation specifications can be significantly enhanced by usingsuch information as: elapsed time since a last reference calibration;empirically determined tolerance of temperature versus offset driftcurves, over time; and elapsed time since power-up. In the above priorart, no means is provided to dynamically and efficiently account forcollective circuit limitations, associated with a drift compensationmethod, in such a way as to provide an accurate, instantaneousindication, or continuous system control, reflecting optimum achievablesystem accuracy, under the drift compensation method, and based uponcurrent operating conditions.

In spite of advances in bridge measurement systems, and in particular,high resolution temperature difference measurement systems, thereremains a need for a high resolution differential thermometer, utilizinga minimum of low cost components, consuming minimal power (permittingbattery powered operation), and operable from a single-ended powersupply, that provides accurate temperature compensation, and that can becalibrated during normal operation, for temperature drift, and timedrift of system components, without significantly interrupting operationin the field, and that sufficiently compensates for time drift ofpassive components, such as thermistors, so that a specified systemresolution, on the order of micro-degrees centigrade, is achievable.Additionally, there remains a need for such a system which provides aninstantaneous indication of system resolution limitations, based uponcurrent operating conditions, which can be reported to the user, oremployed to continuously and automatically effect system reporting insuch a way as to dynamically provide optimal resolution, rather thanusing a single specification based upon a combined estimate of expectedsystem tolerances.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore a general object of the present invention to provide avery low cost means of accurately measuring small signal differences.

It is a more particular object of the present invention to provide avery low cost system for accurately measuring small temperaturedifferences, on the order of micro-degrees centigrade, that can provideaccurate compensation for time and temperature component drift, withoutsignificantly interrupting normal system operation.

It is another object of the present invention to provide a very low costmeans of measuring small temperature differences, on the order ofmicro-degrees centigrade, such that the system can be operated undernormal atmospheric conditions, and calibrated during normal operation,for time and temperature drift of active system components, such asamplifiers and analog to digital converters, and such that saidcalibrations can be readily, and quickly, performed in the field, topermit accurate operation, with resolution on the order of micro-degreescentigrade, without significantly interrupting normal operation.

It is yet another object of the present invention to provide a very lowcost means of measuring small temperature differences, whichsufficiently compensates for time drift of passive components, such asthermistors and feedback resistors, to permit temperature differencemeasurements, with resolution on the order of micro-degrees centigrade,and such that said sufficient compensation can be performed readily, andquickly, in the field, without significantly interrupting normaloperation.

It is still another object of the present invention to provide a verylow cost, high resolution difference measurement system, with continuousindications of measurement system resolution capability, based uponcurrent operating conditions, such that said resolution capability canbe continuously reported to the user or employed to automatically andcontinuously effect system reporting in such a way as to dynamicallyprovide optimum system resolution.

Still other objects and advantages of the present invention will beapparent from the specification which follows.

The present invention improves over prior art systems and techniques formeasuring low level difference signals, such as signals representativeof differential temperature, by reducing the number and cost ofcomponents required to achieve high resolution difference signalmeasurements. The present invention further improves over prior art highresolution difference measurement systems, by permitting accuratecompensation for time and temperature drift of both active and passivesystem components, at a single current arbitrary temperature within arange of ambient temperatures, to permit temperature differencemeasurements, with resolution on the order of micro-degrees centigrade,and such that critical calibrations can be performed withoutinterrupting normal system operation. Additionally, the presentinvention improves over prior art high resolution difference measurementsystems, by providing a continuous indication of measurement systemresolution capability, based upon current operating conditions, andwhich can be continuously reported to the user, or employed toautomatically and continuously effect system reporting in such a way asto dynamically provide optimum system resolution, as opposed to limitingoptimum resolution to a general specification, based upon individualcomponent performance over a temperature range.

Generally, in accordance with the present invention, an improveddifference signal measurement method and apparatus are provided, whichaccurately resolves extremely small differences in electrical signals,in a very low cost, highly portable apparatus that can be batteryoperated, by utilizing predictable behavior in the relative time driftof offset curves, such as thermal offset curves, for various circuitelements of the apparatus, including difference signal amplificationmeans and ambient condition measurement means, such as temperatureamplification means. In an exemplary embodiment, which measurestemperature differences, with resolution on the order of micro-degreescentigrade, the difference signal amplification means includes adifference amplifier, which amplifies a voltage difference between twonodes of a measurement bridge, comprising two thermistors and tworesistors, such that said voltage difference represents a temperaturedifference between said thermistors. Additionally, in the exemplaryembodiment, the ambient temperature amplification means amplifies thedifference in voltage between one of said nodes of said measurementbridge (the voltage of which varies with ambient temperature) and areference node, the voltage of which is provided by a reference bridge(preferably comprising resistors, with substantially time stabletemperature-resistance curves), in order to provide a signalrepresentative of ambient temperature.

In an initial calibration mode (called reference calibration mode, inaccordance with the present invention), preferably performed at the timeof manufacture, thermal offset curves are recorded, which correlateambient temperature measurements to offset measurements from both theambient temperature amplification means and the difference signalamplification means (said measurements are converted to digital form bythe analog to digital converter means). The thermal offset curvesinclude: one curve, recording measurements from the difference signalamplification means, with both thermistors of the measurement bridgeheld at the same temperature, over the ambient temperature range;another curve, recorded with both inputs of the difference signalamplification means held at equal potential, over the ambienttemperature range; and yet another curve, correlating ambienttemperature, over the ambient temperature range, to measurements fromthe ambient temperature amplification means, with both inputs of saidambient temperature amplification means connected to voltages of areference resistor bridge, preferably comprising resistors having asubstantially time stable temperature-resistance characteristic.

One of said thermal offset curves, recorded during operation in thereference calibration mode (said curve referred to as a differencetemperature curve or, more generally, as a physical variable differencecurve, in accordance with the present invention), correlates differencesignal measurements to ambient temperature measurements, with the twothermistors of said measurement bridge held at the same temperature,throughout operation in the reference calibration mode. Consequently,the resulting difference temperature curve can be used, during normaloperation, to correlate any ambient temperature measurement, within therange of calibrated ambient temperatures, to a point on the differencetemperature curve, corresponding to the expected temperature difference,if both thermistors were at the same said measured ambient temperature.That is, during normal operation, a measured ambient temperature can becorrelated to an expected temperature difference measurement(corresponding to zero temperature difference) to provide an offset,which can be used to adjust any measured difference temperature, by saidoffset, in order to compensate for non-matching temperature-resistancecharacteristics, between thermistors, over the ambient temperature rangein which operation in the reference calibration mode was performed. Notethat this does not compensate for drift of active components associatedwith the difference measurements, such as amplifiers and analog todigital converters, which are preferably compensated for as indicatedbelow.

Another thermal offset curve, recorded during operation in the referencecalibration mode, is referred to as a difference reference curve, inaccordance with the present invention, and correlates ambienttemperature measurements to an amplified difference between the twoinputs of the difference signal amplification means of the preferredembodiment, when both said inputs are shorted to the same potential,over the ambient temperature range in which the reference calibrationmode is performed. The difference reference curve is used to compensatefor time drift of the difference temperature curve, over the ambienttemperature range in which the reference calibration mode was performed,and thus is used to compensate for time and temperature drift of activecomponents associated with difference signal measurements, in contrastto drift of passive measurement bridge components, such as thermistors,which typically drift at different rates over time relative to activecomponents, such as amplifiers and analog to digital converters. Notethat the difference reference curve is acquired with both inputs of thedifference signal amplification means shorted to the same potential, incontrast to the difference temperature curve, which is acquired withboth thermistors held at the same temperature, over the ambienttemperature range, in which the reference calibration mode is performed.

Finally, another thermal offset curve, recorded during operation in thereference calibration mode, is referred to as an ambient referencecurve, in accordance with the present invention, and correlates ambienttemperature measurements to an amplified difference between two nodes ofsaid reference bridge, as amplified by the ambient temperatureamplification means. The ambient reference curve is used to compensatefor time and temperature drift of ambient temperature measurements, bytranslating the positions of other thermal offset curves, acquiredduring the reference calibration mode, relative to ambient temperaturemeasurements. Both the ambient reference curve and the ambienttemperature scale, against which all thermal offset curves are measured,are shifted by the method of the present invention, utilizing at leastone measured point and recorded points on the ambient reference curve,along with related measured and recorded ambient temperatures, such thata compensation is achieved for both the ambient temperatureamplification means and passive measurement bridge components,associated with ambient temperature measurements.

The method of the present invention does not require that amplifiers ofeither the ambient temperature amplification means, nor differencesignal amplification means, provide precision performance (such as lowoffset voltage, low temperature drift, low common mode rejection, or, inthe case of battery powered embodiments, in which battery voltage mayvary over time, low power supply rejection). Nor does the method of thepresent invention require that the reference or measurement bridgeemploy time and temperature stable resistors. However, bridge andfeedback resistors of the preferred embodiment of the present inventionare preferably time stable, such as those of standard metal filmcomposition (e.g., manufactured by Dale Electronics, of Norfolk, Nebr.),which offer stability over time, comparable to much more costlytemperature stable, and time stable, bulk metal foil resistors(manufactured by Vishay Electronics Foil Resistors, of Malvern, Pa.), orwirewound resistors (such as manufactured by Dale Electronics, ofNorfolk, Nebr.).

The method of the present invention overcomes time and temperaturecomponent drift, by utilizing the fact that the thermal offset curves,over a given ambient temperature range, drift with time in apredominantly linear fashion, relative to one another, in response tocomponent time drift (e.g., resulting from active component offsetvoltage drift, common mode variations, power supply variations inbattery powered embodiments, and temperature-resistance curve drift,over time, of thermistors and resistors associated with amplifiers andresistance bridges). Hence, the thermal offset curves, representingtemperature drift behavior, which vary among each other in apredominantly linear fashion over time, within a calculable accuracy,are acquired, and stored in memory, so that during normal operation,these curves can be updated for time drift, at a single, currentarbitrary ambient temperature, the measurements for which are obtainedquickly (in a standard calibration mode, in accordance with the presentinvention) and applied as a time drift correction to thermal offsetcurves, without interrupting normal system operation. This method of thepresent invention enables compensation for time and temperature drift ofactive components and passive components sufficiently to permit accurateresolution of temperature differences, on the order of micro-degreescentigrade.

Non-linear time drift, primarily associated with passive component driftover time (e.g., time drift of thermistors and feedback resistors,resulting in gain drift) is substantially compensated by the method ofthe present invention, over many months of operation, without requiringa re-acquisition of thermal offset curves over a temperature range,because such gain drift over time is manifested to a much smaller extentas a change in curve “shape”, than as a linear curve translation.However, such non-linear changes in thermal offset curve shape, overtime, will eventually affect the accuracy of measurements. The extent towhich the effects of non-linear drift over a given time are compensatedcorresponds to a maximum time period, within which re-acquisition ofthermal offset curves is required, in order to achieve a givenmeasurement accuracy. This maximum period depends substantially on thetime stability (not the temperature stability) of passive components,especially gain feedback resistors. In order to determine the maximumperiod between re-acquisition of offset curves that is required in orderto achieve a given accuracy, operation in the reference calibration modeis preferably performed on two occasions, at the time of manufacture,utilizing a constant temperature bath, capable of providing at least twoknown, repeatable temperatures. Nevertheless, the method of the presentinvention permits a re-acquisition of thermal offset curves over anarbitrary temperature range, so that subsequent re-acquisitions ofthermal offset curves can be performed, at any time, by the end user,without such costly calibration equipment.

Additionally, the method of the present invention dynamically trackscumulative system errors, associated with drift compensation, and basedupon current operating conditions, in order to dynamically calculateoptimal system accuracy, based upon current operating conditions (ratherthan based upon a combination of more general component driftspecifications). Once quantified, the calculated optimal systemaccuracy, based upon current operating conditions, can be used todynamically control system reporting, to reflect achievable accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of the present invention will best beunderstood from the following detailed description, taken in conjunctionwith the accompanying drawings, of which:

FIG. 1 is a schematic representation of components comprising a circuitin accordance with one preferred exemplary embodiment of the presentinvention.

FIG. 2 illustrates a graph of the relationships between temperaturecalibration (offset) curves of various components of the circuit of FIG.1, acquired in the reference calibration mode of the method of thepresent invention over an ambient temperature range.

FIG. 3 is a detailed view of the lower left portion of the graph of FIG.2, and illustrates the relationships between reference and temperaturedata points, associated with temperature calibration (offset) curves,acquired from an ambient temperature amplifier, in order to estimatecomponent time drift affecting ambient temperature measurements, inaccordance with the fourth step of the method of the present inventionin the standard calibration mode.

FIG. 4 is a detailed view of the lower left portion of the graph of FIG.2, and illustrates calculations involved in the initial vertical andhorizontal translation of ambient temperature curves, to approximatetime drift compensation for the ambient temperature measurement, inaccordance with the fourth step of the method of the present inventionin the standard calibration mode.

FIG. 5 is a detailed view of the lower left portion of the graph of FIG.2, and illustrates calculations involved in a final determination ofvertical and horizontal translation of ambient temperature calibration(offset) curves, to reflect time drift compensation, for the ambienttemperature measurement, in accordance with the fourth step of themethod of the present invention in the standard calibration mode.

FIG. 6 illustrates the effect that translation error can have on thevertical and horizontal translation of ambient temperature curves, inaccordance with the fourth step of the method of the present inventionin the standard calibration mode, and a method to quantify saidtranslation error.

FIG. 7 illustrates an alternative method to quantify said translationerror, affecting vertical and horizontal translation of ambienttemperature curves, in accordance with the fourth step of the method ofthe present invention in the standard calibration mode.

FIG. 8 is a detailed view of the lower left portion of the graph of FIG.2, and illustrates calculations involved in a determination of timedrift for components, associated with the temperature differencemeasurement in accordance with the fifth, sixth, and seventh steps ofthe method of the present invention in the standard calibration mode.

FIG. 9 is a schematic representation of a preferred external switchingmeans for operating the preferred embodiment of FIG. 1 in the referencecalibration mode, in accordance with the present invention.

FIG. 10 is a detailed view of the lower left portion of the graph ofFIG. 2, and illustrates calculations involved in relating adetermination of current ambient temperature with an initial temperaturedifference measurement, in accordance with the first and second steps ofthe method of the present invention in an operational mode.

FIG. 11 is a detailed view of the lower left portion of the graph ofFIG. 2, and illustrates calculations involved in a determination of thefinal temperature difference measurement, and consolidation of errorsassociated with that measurement, in accordance with the third andfourth steps of the method of the present invention in the operationalmode.

FIG. 12A is a flow diagram representing the steps involved in thepreferred embodiment of the method of the present invention in thereference calibration mode.

FIG. 12B is a flow diagram representing the steps involved in thepreferred embodiment of the method of the present invention in thestandard calibration mode.

FIG. 12C is a flow diagram representing the steps involved in thepreferred embodiment of the method of the present invention in theoperational mode.

FIG. 13 is a schematic representation of components comprising a circuitin accordance with another preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, one exemplary embodiment of the present invention,providing a low cost, high resolution digital temperature differencethermometer, includes: a reference bridge 1; a measurement bridge 2; afirst thermistor 3 and a second thermistor 4, said thermistorsconfigured in parallel, as shown at the upper half of measurement bridge2; reference resistors 5 and 6, comprising reference bridge 1, arrangedas shown, between power rails of measurement bridge 2; a first high gaindifference temperature amplifier 7, with inputs 8 and 9, which can beconnected via a switch 10 to measurement nodes 11 and 12, respectively,of measurement bridge 2, or shorted together such that inputs 8 and 9are both connected to bridge node 11 ; a second high gain differencetemperature amplifier 7 a, with inputs 8 a and 9 a, which can beconnected via a switch 10 a to measurement nodes 12 and 11,respectively, of measurement bridge 2, or shorted together such thatinputs 8 a and 9 a are both connected to bridge node 11; an ambienttemperature amplifier 13, which can be connected via a switch 14 tomeasurement node 11 of measurement bridge 2 and reference node 16 (toamplify the voltage difference between said nodes 11 and 16), or toreference nodes 15 and 16 (to amplify the voltage difference betweensaid nodes 15 and 16), with said reference nodes 15 and 16, formed byreference bridge 1, preferably consisting of time stable, metal filmresistor 5 and wirewound potentiometer 6. It will be appreciated thatthe use of potentiometer 6 is for convenience, and that potentiometer 6may be replaced by a fixed resistor(s), if justified by costconsiderations. Also, note that resistors 5 and 6 need not betemperature stable; however, they are preferably time stable, permittingthe use of standard metal film resistors (e.g., manufactured by DaleElectronics, of Norfolk, Nebr.), which are one to two orders ofmagnitude less costly than time and temperature stable bulk metal foiland wirewound resistors (manufactured by Vishay Electronics FoilResistors, of Malvern, Pa., and Dale Electronics, of Norfolk, Nebr.,respectively), but provide comparable time stability.

As shown in FIG. 1, also included in the exemplary embodiment are:memory means 19; computer means 20; and 8-bit analog to digital (A/D)converter means 17, with at least three input channels (connected to theoutput of high gain difference temperature amplifiers 7 and 7 a andambient temperature amplifier 13); and digital display means 20 a.Computer means 20 and 8-bit A/D converter 17, with at least three inputchannels, are preferably combined within a single, low cost integratedcircuit, such as a Philips 8XC749 microcontroller (manufactured byPhilips Semiconductors, of Sunnyvale, Calif.). Additionally, timingmeans 18, capable of continuous battery powered operation, independentlyof other system components, determines elapsed time since the mostrecent reference calibration, and may also be used to determine elapsedtime during system warm-up. Memory means 19 contains calibration data,as well as other data, used by computer means 20, with output from A/Dconverter 17 and timing means 18, to compensate for system time andtemperature drift, while determining temperature differences betweenthermistors 3 and 4, as well as computing optimal system resolutionlimitations, responsive to current operating conditions.

In the embodiment of FIG. 1, measurement bridge 2 is powered by bridgevoltage 21 (e.g., a battery), with ratiometric bridge output 22,routinely used in the art with A/D converters, so that variations inbridge voltage 21 are compensated in A/D converter 17. Alternatively, itwill be appreciated by those skilled in the art that A/D converter 17itself may output a reference signal voltage which can alternatively beused to provide bridge voltage 21.

With the resistance of time stable, wirewound, left bridge potentiometer23 (time stable, metal film resistors may be substituted, depending oncost/accuracy trade-offs), substantially equal to the resistance of timestable, metal film, right bridge resistor 24, and the nominal resistanceof left bridge thermistor 3 substantially equal to the nominalresistance of right bridge thermistor 4, then any temperature differencebetween thermistor 3 and thermistor 4 will appear as a small voltagedifference between bridge nodes 11 and 12. Difference temperatureamplifiers 7 and 7 a can be connected to measurement bridge 2, viaswitches 10 and 10 a, respectively, at nodes 11 and 12, as shown in FIG.1, so that the voltage potential difference between nodes 11 and 12,corresponding to any said temperature difference between thermistors 3and 4, is amplified and further processed in accordance with the presentinvention to provide a temperature difference measurement. Furthermore,since the preferred embodiment operates from a single power supply, inorder to insure that a positive difference voltage between bridge nodes11 and 12 is available for amplification under all conditions,difference temperature amplifiers 7 and 7 a are connected to bridgenodes 11 and 12, with opposite polarity, as shown, and bridgepotentiometer 23, and/or amplifier offset voltages are trimmed, suchthat at least one positive difference voltage between bridge nodes 11and 12 is always available, throughout the expected ambient temperaturerange.

In the embodiment of FIG. 1, at least one of the thermistors 3 and 4 isalso used to determine system ambient temperature, using ambienttemperature amplifier 13, at substantially lower gain than that ofdifference temperature amplifiers 7 and 7 a, said gain preferably beingdetermined so that a useful span of ambient temperatures can be measuredwithout clipping the ambient temperature signal. Over any given ambienttemperature range, difference temperature (and other thermal offsetcurve) measurements of the embodiment of FIG. 1 will typically vary twoto three orders of magnitude more slowly than ambient temperaturemeasurements. This permits the gain of ambient temperature amplifier 13to be two to three orders of magnitude lower than the gain of differencetemperature amplifiers 7 and 7 a, in order to achieve a giventemperature difference measurement resolution. Consequently, it will beappreciated by those skilled in the art that the amplification gain ofambient temperature amplifier 13 may be one (1), for example, such thatambient temperature amplifier 13 operates as a voltage follower, or,alternatively, such that ambient temperature amplifier 13 may actuallyattenuate the signal representative of ambient temperature, i.e.,possess an amplification gain of less than one (1). It will also beappreciated by those skilled in the art that in the case where a gain ofone (1) is sufficient for the signal representative of ambienttemperature, it is conceivable that the ambient temperature amplifier 13can be eliminated, and the output of switch 14 can be connected directlyto A/D converter 17, bypassing ambient temperature amplifier 13 in FIG.1, such that switch 14 can be operated to connect A/D converter 17 to asignal from measurement node 11, representative of ambient temperature,or to reference node 15 for the purpose of generating offset curvesassociated with ambient temperature measurement. Additionally, it willbe appreciated by those skilled in the art that since, as mentionedabove for the preferred embodiment of FIG. 1, ambient temperaturemeasurements can be made with lower gain than difference measurements inorder to achieve a given temperature difference measurement resolution,offset errors associated with ambient temperature measurements have asmaller effect than offset errors associated with differencemeasurements, so that the above said offset curves associated withambient temperature measurement, that would otherwise be used tocompensate said errors associated with ambient temperature measurement,may conceivably be unnecessary to achieve a given temperature differencemeasurement resolution, in which case switch 14 may also be eliminated,such that a signal from measurement node 11, representative of ambienttemperature, is connected directly to A/D converter 17, without the needto connect A/D converter 17 to reference node 15 for the purpose ofgenerating the said offset curves associated with ambient temperaturemeasurement. In any case, whether an ambient temperature amplifier 13 ora switch 14 is included in the circuit path between a signalrepresentative of ambient temperature and A/D converter 17, the meansthrough which a signal representative of ambient temperature is providedto A/D converter 17 is more generally referred to as the ambientcondition signal means.

Ambient temperature amplifier 13 also receives a reference signal,preferably in the form of a reference voltage from reference node 16,which is determined by resistors 5 and 6 of reference bridge 1, suchthat ambient temperature amplifier 13 amplifies the voltage differencebetween nodes 11 and 16, or the difference between nodes 15 and 16(depending on the state of switch 14), and with said resistors 5 and 6chosen so that ambient temperature amplifier 13 realizes the greatestpossible gain, and range, without clipping output signals, over theexpected operating temperature span of the system. Thus, in thepreferred embodiment, the gain of ambient temperature amplifier 13 ispreferably fixed, and the reference voltage from reference node 16 isalso preferably fixed. Nevertheless, those skilled in the art willrecognize that the gain of ambient temperature amplifier 13, and/or thevoltage of reference node 16, may economically be made adjustable, toincrease the achievable gain and input range of ambient temperatureamplifier 13, if desirable. The output signals of difference temperatureamplifiers 7 and 7 a and ambient temperature amplifier 13 are convertedfrom analog to digital form, using A/D converter 17, so that thesesignals are available for further processing by computer means andmemory means 19.

In one modification of the embodiment shown in FIG. 1, substantiallybalanced bipolar power can be supplied to reference bridge 1 andmeasurement bridge 2. Accordingly, it will be appreciated by thoseskilled in the art that bridge resistances can be selected such thatvoltages from nodes 15 and 16 of reference bridge 1 and voltages fromnodes 11 and 12 of measurement bridge 2 may be approximately zero volts(ground). Consequently, with node 16 at ground potential, a minimalvoltage difference between said node 16 and node 11, as measured byambient temperature amplifier 13 with switch 14 operated to connect saidnode 11 to ambient temperature amplifier 13, is obtained over a givenambient temperature range, thus maximizing the possible amplificationfactor which can be provided by ambient temperature amplifier 13 withoutexceeding its output voltage limitations, so as to avoid clipping theoutput signal of ambient temperature amplifier 13 at ambienttemperatures within the said ambient temperature range. It will also beappreciated by those skilled in the art that nodes 15 and 16 canalternatively be shorted together and connected to a single referencesignal, such as system ground, or that the reference signals from nodes15 or 16 can alternatively be provided by a reference signal source,such as a digital to analog converter. Those skilled in the art willrecognize that in the case in which nodes 15 and 16 are shortedtogether, the reference bridge resistances 5 and 6 become unimportantand, consequently, can conceivably be omitted.

As described above, low cost, time stable metal film resistors may beused in place of more expensive time and temperature stable resistors inthe preferred embodiment of FIG. 1. Additionally, in order to facilitatelow system cost, difference temperature amplifiers 7 and 7 a and ambienttemperature amplifier 13 and other circuit components need not beoptimized for low drift, low input offset voltage, low common moderejection (CMR), or, in the case of battery powered embodiments, inwhich battery voltage may vary over time, low power supply rejection(PSR). Neither is it necessary that nodes 11 and 12 be maintained nearzero volts, in the interest of minimizing amplifier CMR, as is often thecase in prior art, high resolution difference measurement systemsutilizing a bridge circuit, which consequently also resort to the addedexpense and components of a bipolar supply voltage, or other costlyadditions to the measurement bridge. In the preferred embodiment of thepresent invention, operational amplifiers, such as those used fordifference temperature amplifiers 7 and 7 a and ambient temperatureamplifier 13, bipolar power, thermal drift, offset voltage, PSR, and CMRrequirements need not limit system design, permitting full attention tobe paid during design, and component selection, to minimizing amplifiernoise, and cost.

A preferred embodiment of the present invention (utilizing low costamplifiers, such as OPA2234, manufactured by Burr-Brown Corporation, ofTucson, Ariz.) compensates for offset and gain drift due to componenttime and temperature drift, without requiring the use of expensiveprecision components, by operating in three modes, according to thepreferred embodiment of the method of the present invention, which serveto compensate for both time and temperature drift, of active and passivecomponents, including drift of amplifier CMR with time and temperature.These modes include a reference calibration mode, a standard calibrationmode, and an operational mode, as described in greater detail below.

Generally, the reference calibration mode is performed at least once,following initial manufacture, and generates a set of thermal offsetcurves, specific to a particular system. These thermal offset curvesmeasure system offset voltages versus ambient temperature, over theambient temperature span in which the system is expected to operate.

The standard calibration mode is preferably performed once, each timethe system is turned on, and corrects the thermal offset curvesgenerated during the reference calibration mode for time drift, with fewpre-programmed measurements, that can be performed quickly, withoutadditional equipment, at the outset of a measurement session. If thestandard calibration mode is not performed during one or more particularsessions, in which the system is in continuous operation, the system canautomatically reduce resolution to anticipate potentially resultingerrors that are calculable, based upon data acquired during previousoperation of the system in the standard calibration mode.

In the operational mode, the embodiment of FIG. 1 continuously measuresambient temperature, using thermal offset curves corrected during thestandard calibration mode for time and temperature drift, and correlateseach actual ambient temperature measurement, acquired during a normaloperation run, to an expected temperature difference measurement, ifboth thermistors were at the same temperature (also corrected, duringoperation in the standard calibration mode, for component drift). Saidexpected temperature difference measurement, if both thermistors were atthe same temperature, serves as an offset to an associated actualtemperature difference measurement, correcting said actual temperaturedifference measurement for the effects of component time and temperaturedrift, at the current ambient temperature. Additionally, during theoperational mode, while performing measurements of temperaturedifference between thermistors 3 and 4, the preferred embodiment of thepresent invention automatically determines, for each measurement, theoptimum achievable system accuracy, based upon current operatingconditions.

Each of the above three modes of operation has a specific function inthe method in accordance with the present invention, which is firstgenerally described below, for each mode. A complete description of eachmode of operation follows these general descriptions.

Referring to FIGS. 1 and 2, the reference calibration mode (RCM) recordsdata points in memory 19, corresponding to the various reference andtemperature difference measurements performed by the embodiment of FIG.1 versus ambient temperature (as amplified by ambient temperatureamplifier 13), over the range of ambient temperatures in which thesystem is expected to operate. More specifically, these data points formcurves, defining relationships between voltages representing ambienttemperature, measured by ambient temperature amplifier 13, and thefollowing:

a) a reference measurement from nodes 15 and 16, connected to ambienttemperature amplifier 13 (corresponding to curve 26 in FIG. 2);

b) a reference measurement from bridge node 11, shorted to both inputs 8and 9 of difference temperature amplifier 7 (via switch 10 in FIG. 1,and corresponding to curve 28 in FIG. 2);

c) another reference measurement from bridge node 11, shorted to bothinputs 8 a and 9 a of difference temperature amplifier 7 a (via switch10 a in FIG. 1, and corresponding to curve 28 a in FIG. 2); and

d) two temperature difference measurements, one for each of thedifference temperature amplifiers 7 and 7 a, representing a voltagedifference between bridge nodes 11 and 12 over the range of temperaturesin which the system is expected to operate (corresponding to curves 27and 27 a, respectively, in FIG. 2).

It is important to note that during RCM, both thermistors 3 and 4(FIG. 1) are held at substantially the same temperature. This may beaccomplished, for example, by holding thermistors 3 and 4 in closeproximity or physical contact and/or in a homogeneous thermalenvironment, such as a constant temperature bath. The importance ofholding thermistors 3 and 4 at substantially the same temperature duringRCM relates to the fact that variance between thermistorresistance-temperature curve characteristics, even in well matchedthermistors, will result in a variation in the measured temperaturedifference, between thermistors 3 and 4, over an ambient temperaturerange, even if the thermistors are held at exactly the same temperature.Nevertheless, it should be noted that variance between thermistorresistance-temperature curve characteristics occurs extremely slowlyover ambient temperature. For example, if low cost, moderately matchedthermistors are used, and said thermistors are held at the same ambienttemperature, an ambient temperature change of 1° C. will correspond to avariation in temperature difference measurement of less than a fewmilli-degrees, but clearly, this is a significant amount in temperaturedifference measurements, with desired resolution on the order ofmicro-degrees centigrade. In part, to compensate for this effect, RCMgenerated curves 27 and 27 a (FIG. 2) represent thermistor differencetemperatures, corresponding to a zero temperature difference, over theambient temperature range in which RCM is performed. During normaloperation, these curves 27 and 27 a are used to correlate a said zerotemperature difference, as an offset voltage, to each measuredtemperature difference measurement, with said offset determined bymeasuring ambient temperature, and associating the measured ambienttemperature with a corresponding said zero temperature differenceoffset, on curve 27 and/or 27 a. That is, during RCM, with thermistors 3and 4 at substantially the same temperature, any deviation from a zerotemperature difference measurement, at a given ambient temperature, isan offset to the difference measurement, at said ambient temperature,which during normal operation is used to adjust measured temperaturedifferences (at said ambient temperature) in order to compensate fornon-matching temperature-resistance characteristics between thermistors3 and 4, over the ambient temperature range in which operation in RCMwas performed.

The present invention recognizes the fact that relationships between RCMgenerated curves drift over time, with respect to ambient temperature,in a predominantly linear fashion, to within a calculable accuracy.Consequently, though RCM generated curves are preferably acquired bycycling components of the embodiment of FIG. 1 through an expectedoperating range of ambient temperatures, said RCM generated curves needonly be acquired once, or infrequently, when using resistive componentswhose temperature-resistance curves are substantially time stable (incontrast to resistance being stable with temperature, which is notnecessary). Additionally, the RCM generated curves can be updated toreflect linear component drift over time, with a single measurement, inthe standard calibration mode (described in more detail below), whichcan be performed almost instantly, in the field. To a high degree, gaindrift over time, associated with the time drift of resistors andthermistors, is also substantially compensated by this method, sincesaid gain drift is largely manifested as a linear translation of offsetcurves, and to a much smaller degree as a change in curve “shape”. Thatis, linear translation of RCM generated offset curves, by the method ofthe present invention, substantially compensates both linear offsetcurve drift, and drift of passive components, normally associated withgain drift. The distinction between linear and non-linear curve drift,and how each is dealt with in the method of the present invention, isdiscussed in detail below, during the detailed description of thestandard calibration mode (fourth step 213 in FIG. 12B). During normaloperation, ambient temperature, and temperature difference measurements,acquired during the operational mode, also described in more detaillater, proceed, utilizing RCM generated curves that have been adjustedfor time drift during the standard calibration mode, such that normaloperation proceeds without interruption for further time or temperaturedrift compensation. Errors associated with the above process arecalculable, and vary with operating conditions, such as elapsed timesince the most recent acquisition of RCM curves, current ambienttemperature, measured curve trends, and elapsed time since power wasapplied to the system, which, once quantified, can be used todynamically control system reporting to reflect accuracy limitations, asthey change with the above system operating conditions.

The standard calibration mode (SCM) performs the function of adjustingthe above acquired RCM generated curves for variation over time,preferably once, for each separate session during which the system isoperated. SCM compensates for errors resulting from time drift of systemcomponents, by observing variations in the relationships of the RCMgenerated curves. This SCM compensation function can be performed at anysingle ambient temperature, and results in six data points beingacquired, each said data point corresponding to a measurement for eachof the six RCM generated curves, and such that each said data point iscompared to its corresponding data points on previously acquired RCMgenerated curves.

Thermal offset curves associated with ambient temperature measurements(RCM generated curves 25 and 26 in FIG. 2) are substantially compensatedfor time drift, as a result of operation in SCM, at an arbitrary ambienttemperature, by utilizing a measured ambient temperature point,corresponding to said arbitrary ambient temperature, positioned on theRCM generated ambient temperature curve 25, and then comparing acorresponding point to said measured ambient temperature point, saidcorresponding point situated on RCM generated ambient reference curve26, with an actual measurement at said arbitrary ambient temperature,from ambient temperature amplifier 13, with switch 14 configured toconnect both inputs of ambient temperature amplifier 13 to nodes 15 and16 of reference bridge 1. The above actual and recorded measurementpoints, together with other point-to-point relationships between RCMgenerated curves 25 and 26, are used to effect a shift in the ambienttemperature scale, relative to other RCM generated curves, includingcurves 27, 27 a, 28, and 28 a. The result of said shift is that theambient temperature scale, against which all said thermal offset curvesare measured, is shifted by the method of the present invention,utilizing the above mentioned points on both ambient reference curve 26and ambient temperature curve 25, such that a substantial compensationis achieved for the ambient temperature amplification means, comprisingambient temperature amplifier 13 and associated feedback resistors, A/Dconverter 17, and passive measurement bridge components associated withthe ambient temperature measurement, including resistors andthermistors.

Additionally, during operation in SCM, components associated withtemperature difference amplification, including difference temperatureamplifiers 7 (and 7 a), and associated feedback resistors, aresubstantially compensated for drift, by comparing previously recordedpoints associated with the current said arbitrary ambient temperature(as measured on the ambient temperature scale, adjusted as abovedescribed) on the RCM generated difference reference curve 28 (and 28a), with a point acquired with switches 10 (and 10 a) configured tore-acquire said point at the current ambient temperature (i.e., withswitches 10 and 10 a configured to connect all inputs of differencetemperature amplifiers 7 and 7 a to measurement bridge node 11).Combined with measurements at the current ambient temperature, thatre-acquire points on the RCM generated difference temperature curve 27(and 27 a), and calculations that compare said re-acquired points withpreviously recorded points on said RCM generated difference temperaturecurve 27 (and 27 a), at the current ambient temperature (as measured onthe ambient temperature scale, adjusted as above described), the abovedescribed comparisons and calculations, involving RCM generated curves28 (and 28 a), and 27 (and 27 a), are used to substantially compensatefor component drift associated with the temperature differencemeasurements. Once again, it is important to note that during operationin SCM, both thermistors 3 and 4 (FIG. 1) are held at substantially thesame temperature. The importance of holding thermistors 3 and 4 atsubstantially the same temperature during SCM is that the RCM generatedcurves 27 and 27 a (FIG. 2) represent a zero difference temperaturebetween thermistors, over the ambient temperature range in which RCM wasperformed, as described above. Therefore, in order for operation in SCMto adjust curves 27 and 27 a at a given measured ambient temperature toreflect updated values for said curves 27 and 27 a, at said measuredambient temperature, said operation in SCM must recreate the conditionof zero temperature difference between thermistors 3 and 4, as when saidcurves 27 and 27 a were initially acquired during operation in RCM.

During operation in SCM, RCM generated curves are corrected for lineartime drift to within a quantifiable accuracy, described later in termsof an error quantity, referred to as translation error. SCM alsodynamically calculates error tolerance from error sources, such astranslation error, that are responsive to current operating conditions,such as ambient temperature, trends in translation error over time, andelapsed time since the last operation in RCM, so that estimates ofsystem accuracy limitations are always optimized, during the operationalmode, based upon current operating conditions, rather than being basedupon generalized component drift specifications for the applicabletemperature range. Finally, SCM tracks trends in component drift, sothat sources of error related to random effects can be separated fromsources of error which reflect a consistent shift in RCM generatedcurves, so that said RCM generated curves can be further compensated.

Primarily, the operational mode (OM) uses SCM time drift correctedcurves to dynamically calculate temperature differences (betweenthermistors 3 and 4) to perform a high resolution differentialtemperature measurement. Specifically, each current ambient temperaturemeasurement is associated with an accompanying temperature differencemeasurement, which, in turn, is additionally associated with an expectedzero temperature difference measurement, reflecting the expectedmeasured temperature difference if thermistors 3 and 4 were both held atsaid current ambient temperature. The expected zero temperaturedifference for the current ambient temperature is then effectively usedto adjust the temperature difference measurement, acquired at thecurrent ambient temperature, as a difference measurement offset, inorder to accurately report the temperature difference betweenthermistors 3 and 4. Additionally, OM uses dynamic error tolerancescalculated during SCM to automatically control reporting of temperaturedifferences, such that said reporting reflects achievable accuracy, thatis dynamically updated to reflect current operating conditions.

Reference Calibration Mode (RCM)

Referring to the graph of FIG. 2, RCM of the preferred embodiment of themethod of the present invention serves as the initial step incompensating for offset and gain drift of system components due to timeand temperature, by generating curves 25, 26, 27, 27 a, 28, and 28 aover a range of measured ambient temperatures, and correlating thediscrete calibration points on these curves to said measured ambienttemperatures, at which they were acquired. That is, each of the RCMgenerated curves 25, 26, 27, 27 a, 28, and 28 a represents a measuredvoltage (vertical axis) versus measured voltage of ambient temperature(horizontal axis), the latter as measured by ambient temperatureamplifier 13 (FIG. 1). The embodiment shown in FIG. 1 is preferablyoperated in RCM, at least once, after manufacture. Subsequent operationin RCM may optionally be performed, as desired, to augment the morefrequent operation in SCM (described in more detail later). It is alsoconceivable that curves generated by RCM may be acquired once for manysystems (e.g., in which analog components are “matched” by themanufacturer), and hard-coded into memory 19 (FIG. 1), for all suchdevices. However, unless RCM curves are generated for a specific system,this would result in inaccuracy resulting from minor differences betweenidentical components, in different systems, thus limiting achievableaccuracy. Additionally, it is conceivable that individual system driftparameters may be hard-coded into memory 19 (FIG. 1) in order tocharacterize the performance of system components, such as thermistors,resistors, and amplifiers, over an intended operating temperature range.For example, thermistor resistance curves over temperature can behard-coded into memory 19 (FIG. 1), using coefficients routinelyspecified for thermistors (see, for example, Philips Components DataHandbook PA02, 1995, page 75) or empirically determined for one or boththermistors 3 and 4, in order to permit a degree of interchangeabilitybetween thermistors used in similar systems, so that the behavior of oneor both thermistors 3 and 4 can be anticipated using said coefficients,said anticipated behavior being applied along with the effects of othercircuit components in order to generate said RCM curves, withoutnecessarily requiring that the system be operated in RCM in order toaccommodate particular thermistors.

Referring to FIGS. 1 and 2, in RCM the embodiment of FIG. 1 ispreferably cycled through an expected range of ambient temperatures.Each said ambient temperature is measured, with switch 14 operated toconnect ambient temperature amplifier 13 to bridge node 11, and for eachambient temperature measurement, a point is recorded (in memory 19) onambient temperature curve 25 (FIG. 2). Additionally, for each saidrecorded point on ambient temperature curve 25, another measurement isrecorded in memory 19 for each of the following:

a) ambient temperature amplifier 13, connected to reference nodes 15 and16 (corresponding to ambient reference curve 26);

b) difference temperature amplifier 7, connected to bridge nodes 11 and12 (corresponding to first difference temperature curve 27);

c) difference temperature amplifier 7, with both inputs connected tobridge node 11 (corresponding to first difference reference curve 28);

d) difference temperature amplifier 7 a, connected to bridge nodes 11and 12 (corresponding to second difference temperature curve 27 a); and

e) difference temperature amplifier 7 a, with both inputs connected tobridge node (corresponding to second difference reference curve 28 a).

Referring to FIG. 2, curves 27 and 28 are approximately mirrored bycurves 27 a and 28 a, respectively, about the horizontal axis. Thisreflects the way that difference temperature amplifiers 7 and 7 a areconnected to bridge nodes 11 and 12, with opposite polarity, as shown inFIG. 1. The intent of this, as briefly mentioned above, is to permit atleast one of the difference temperature amplifiers 7 and 7 a to alwaysamplify a positive difference voltage. Additionally, to insure thatoffset errors for difference temperature amplifiers 7 and 7 a are suchthat a positive difference voltage is always available from at least oneof amplifiers 7 and 7 a, said amplifiers 7 and 7 a are preferablymanufactured on a single silicon substrate (e.g., as a “dual” op amp,such as OPA2234, manufactured by Burr-Brown Corporation, of Tucson,Ariz.). An additional measure, to insure that at least one of thedifference temperature amplifiers 7 and 7 a always provides a positivedifference voltage, is to introduce an artificial offset to saiddifference temperature amplifiers using an offset adjustment techniquefor operational amplifiers, which is well known to persons skilled inthe art. Another conceivable alternative is to replace differencetemperature amplifiers 7 and 7 a with a single bipolar (positive andnegative supply) amplifier, which would also necessitate the use of abipolar A/D converter 17 and conceivably include bipolar power tomeasurement bridge 2 and reference bridge 1.

To summarize, referring to FIG. 2, ambient temperature curve 25 consistsof measured ambient temperature calibration points, each correlated to acorresponding point on ambient reference curve 26; and to a point oneach of the difference temperature curves 27 and 27 a; as well as to apoint on each of the difference reference curves 28 and 28 a. Hence, toeach ambient temperature calibration point generated in RCM on ambienttemperature curve 25, five other points are correlated, one point foreach of the other five RCM generated curves (26, 27, 27 a, 28, and 28a), as shown in FIG. 2.

Referring to FIG. 2, in the preferred embodiment of the method of thepresent invention, measurement points for RCM generated curves areacquired over a range of discrete ambient temperatures, in the rangeT_(amb0)-T_(amb25), corresponding to twenty-five equally spacedincrements, each said increment corresponding to ten output levels ofA/D converter 17 (FIG. 1). These output levels are measured in leastsignificant bits (LSB's) of A/D converter 17, said A/D converter 17having a full range of two-hundred and fifty-six LSB's, though forsimplicity, in the preferred embodiment, only two-hundred and fifty ofsaid LSB's are used for measuring ambient temperature. RCM is preferablyperformed in two phases: a measurement phase, followed by an analysisphase, both described in detail below.

Referring to FIG. 2, each of the following RCM steps of the RCMmeasurement phase, described below, is repeated in the followingsequence, at each calibration temperature, each said calibrationtemperature associated with an ambient temperature calibration point onambient temperature curve 25. The calibration temperatures are separatedfrom one another by one reference ambient temperature increment (RATI),preferably defined as ten LSB's of A/D converter 17 (a typical RATI isidentified as 29 in FIG. 2). However, it will be appreciated by thoseskilled in the art that the number of LSB's of A/D converter 17associated with one RATI can be increased or reduced, with a resultingreduction or improvement in measurement accuracy, respectively. A flowdiagram, generally representing each of the individual steps involved inoperation of the preferred embodiment of the present invention in thereference calibration mode, is shown in FIG. 12A.

RCM Step 1 (201 in FIG. 12A): Referring to FIG. 1, switch 14 isoperated, preferably by computer means 20, to connect ambienttemperature amplifier 13 to bridge node 11, and the resulting signalfrom ambient temperature amplifier 13 is converted to digital form byA/D converter 17 and stored in memory 19, as the ambient temperaturemeasurement to be associated with subsequent RCM measurements (RCM steps2-4, corresponding to 203-205 in FIG. 12A, below), to be acquired at thepresent ambient temperature. Referring to FIG. 2, the curve eventuallygenerated by points derived over successive operations in RCM, due tothis RCM step 1 (210 in FIG. 12A), is hereafter referred to as the RCMgenerated ambient temperature curve 25.

RCM Step 2 (203 in FIG. 12A): Referring to FIG. 1, the offset of ambienttemperature amplifier 13 at the present ambient temperature is nextmeasured. To effect this measurement, switch 14 is operated, preferablyby computer means 20, to connect reference voltages 15 and 16,determined by time stable resistors 5 and 6, to the inputs of ambienttemperature amplifier 13. The resulting reference voltage measurementfrom ambient temperature amplifier 13 is converted to digital form byA/D converter 17 and stored in memory 19, as being associated with thecurrent ambient temperature measurement, derived above, in RCM step 201.Referring to FIG. 2, the curve eventually generated by points derivedover successive operations in RCM, due to this RCM step 203, ishereafter referred to as the RCM generated ambient reference curve 26.In the modified embodiment in which nodes 15 and 16 are shorted togetheras described earlier, whether said nodes 15 and 16 are connected to asingle reference signal, such as system ground, or left floating, andswitch 14 is operated to connect ambient temperature amplifier 13 toreference node 15, as above described for this RCM Step 2, both inputsto ambient temperature amplifier 13 are consequently shorted together,and ambient reference curve 26 is acquired in this configuration.

RCM Step 3 (204 in FIG. 12A): Next, referring to FIG. 1, switches 10 and10 a are operated, preferably by computer means 20, to connect bothinputs (8 and 9) of difference temperature amplifier 7, and both inputs(8 a and 9 a) of difference temperature amplifier 7 a, respectively, tobridge node 11, so that only offset voltages resulting from differencetemperature amplifiers 7 and 7 a and A/D converter 17 for the currentambient temperature measurement will be represented. These offsetvoltages are stored in memory 19 and associated with the current ambienttemperature measurement, derived above, in RCM step 201. Referring toFIG. 2, the curve eventually generated, by points derived oversuccessive operations in RCM, due to this RCM step 204, associated withdifference temperature amplifier 7, is hereafter referred to as thefirst RCM difference reference curve 28. The curve eventually generatedby points derived over successive operations in RCM (due to this RCMstep 204), associated with difference temperature amplifier 7 a, ishereafter referred to as the second RCM difference reference curve 28 a.Note that when both inputs to difference temperature amplifier 7 or 7 aare shorted to a single potential, or when both inputs to ambienttemperature amplifier 13 are shorted to a single potential, for example,as described above in connection with RCM step 203, the said singlepotential is more generally referred to as a common signal.

RCM Step 4 (205 in FIG. 12A): Finally, referring to FIG. 1, switches 10and 10 a are operated, preferably by computer means 20, to reconnect thetwo inputs 8 and 9 of difference temperature amplifier 7, and the twoinputs 8 a and 9 a of difference temperature amplifier 7 a,respectively, to separate bridge nodes 11 and 12, such that bothdifference temperature amplifiers 7 and 7 a amplify the voltagedifference between said bridge nodes 11 and 12. The resulting differencevoltages from difference temperature amplifiers 7 and 7 a, after beingconverted to digital form by A/D converter 17, are then stored in memory19 and associated with the current ambient temperature measurement,derived above, in RCM step 201. Note that the first differencetemperature amplifier 7 is configured to amplify the voltage differenceof node 11, minus that of node 12, while the second differencetemperature amplifier 7 a is configured to amplify the voltagedifference of node 12, minus that of node 11, to insure the consistentavailability of a positive difference voltage, as described above.Referring to FIG. 2, the curve eventually generated by points derivedover successive operations in RCM, due to this RCM step 205, associatedwith the first difference temperature amplifier 7, is hereafter referredto as the first RCM difference temperature curve 27. The curveeventually generated by points derived over successive operations in RCM(due to this RCM step 205), associated with the second differencetemperature amplifier 7 a, is hereafter referred to as the second RCMdifference temperature curve 27 a.

As briefly described above, both thermistors 3 and 4 (FIG. 1) areconfigured to experience substantially the same temperature throughoutthe RCM operation. This is preferably accomplished by placingthermistors 3 and 4 in an assembly which mechanically places themtogether during RCM, in such a way as to facilitate good thermalcontact, preferably packed within a thermally conducting sleeve, withsaid sleeve surrounding, and in physical contact with, both thermistors3 and 4, so that they experience substantially the same temperature. Thequality of this contact will, in part, determine the error implicit insubsequent difference measurements between thermistors 3 and 4. Forexample, if the maximum possible temperature differential betweenthermistors 3 and 4, while in thermal contact during RCM and due to lessthan perfect thermal contact, is one micro-degree centigrade, then onemicro-degree centigrade will be a practical limit to reliabletemperature compensation, in subsequent temperature differencemeasurements, based upon these calibrations. A conceivable alternative,to enhance or replace physical thermal contact during operation in RCM,is to place the embodiment of FIG. 1 in a substantially homogeneousthermal environment, such as a constant temperature bath during RCM, andslowly ramp the temperature of the bath, from one end of the expectedambient temperature range to the other.

Referring to FIG. 1, in order to increment the temperatures at which RCMcalibration points are acquired, computer means 20 monitors the outputfrom ambient temperature amplifier 13 until a specified RCM generatedambient temperature increment (RATI, e.g., 29 in FIG. 2) exists betweenthe last ambient temperature measurement and a current ambienttemperature measurement. Note, as mentioned above, that one RATI in FIG.2 is designated to be ten LSB's of A/D converter 17, as indicated on thehorizontal axis of FIG. 2. When said increment of one RATI existsbetween the last ambient temperature measurement and a current ambienttemperature measurement, the RCM process is repeated (beginning at RCMstep 201, as indicated at 202 in FIG. 12A), such that each time the RCMprocess is repeated, a new set of six calibration points is acquired,one said calibration point for each of the six RCM generated curves 25,26, 27, 27 a, 28, and 28 a in FIG. 2, at said current ambienttemperature.

In order to determine when to terminate the RCM process, computer means20 (FIG. 1) uses timer 18 to determine if sufficient time has elapsedsince the last RCM generated ambient temperature measurement, duringwhich ambient temperature has not incremented by at least one RATI 29(FIG. 2), to indicate that the current ambient temperature is themaximum currently available. If this is the case, computer means 20terminates the measurement phase of RCM.

Referring to FIG. 2, note that vertical axis units (LSB's) are the sameas horizontal axis units, and that the horizontal axis is a measurementof the current ambient temperature (also represented by ambienttemperature curve 25), and the vertical axis measures each of the curves25, 26, 27, 27 a, 28, and 28 a at said current ambient temperature.Referring to FIG. 1, these curves are related to ambient temperature, asin the following example. In order to know the value of the expectedambient reference measurement (from ambient temperature amplifier 13when said amplifier is connected via switch 14 to node 15) on ambientreference curve 26 in FIG. 2, at an ambient temperature, indicated bythe dashed line 38 a, on the horizontal axis (at two RATI's, or twentyLSB's of AID converter 17), one can draw a vertical line through 38 a,as shown, to the intersection with ambient reference curve 26, and atsaid intersection, draw a horizontal line to point 38 b, on the verticalaxis, as shown (corresponding to a measurement of roughly fifty-oneLSB's of A/D converter 17, as shown). Hence, referring to FIG. 2, theRCM ambient reference measurement, correlated with ambient temperature38 a, is 38 b. Similarly, each of the curves 26, 27, 27 a, 28, and 28 acan be associated with an ambient temperature measurement.

After initially operating the system in RCM, following manufacture, RCMmay conceivably be performed in the field, as desired to occasionallyaugment SCM (described in more detail below), without the use of atemperature controlled environment (such as a constant temperaturebath). This is preferably accomplished by allowing the embodiment ofFIG. 1 to cool in a readily available cold environment (e.g., arefrigerator, or melting ice) and subsequently allowing it to warm toroom, or other available, ambient temperature. These cooling and warmingprocesses can conceivably be conducted by placing the embodiment of FIG.1 in a box that is moderately insulated, permitting said embodiment tocool and warm more slowly and uniformly.

During the analysis phase of RCM, computer means 20 (FIG. 1) uses datapoints generated for the six curves, in the measurement phase of RCM, tobetter describe these curves, so that interpolations between discretepoints can readily be made during normal operation, in order to minimizecalculations necessary during normal operation, and to facilitate moreexact measurements. The simplest approach is to organize only theexisting calibration points for each of the six curves, so that thevoltage associated with any ambient temperature measurement (on ambienttemperature curve 25 in FIG. 2) is readily correlated to its fiveassociated points on the other five curves. Then during normaloperation, any intermediate point, between two calibration points for agiven curve, acquired during RCM, can readily be interpolated linearly,using the equation of the line defined by the said two calibrationpoints, surrounding said intermediate point. Alternatively, a polynomialexpansion may be created during the analysis phase of RCM, for each ofthe six curves, using a curve fitting method, such as the least squaresmethod, well known to persons skilled in the art, to fit a curve todiscrete data.

Achievable system accuracy depends in part on which of the availableanalysis methods (e.g., linear interpolation or polynomial expansion) isused, and the number of points involved. In the following discussions,linear interpolations are used to describe points along curves, situatedbetween measured calibration points. This results in a linearinterpolation error, distinct from such errors associated with othercurve fitting methods. Accordingly, the method of the present inventionpreferably strives to take a sufficient number of calibration points tominimize, or negate, the effects of linear interpolation error.

Standard Calibration Mode (SCM)

Referring to FIG. 2, the preferred embodiment of the method of thepresent invention in SCM adjusts the positions of the above describedRCM generated curves 26, 27, 27 a, 28, and 28 a relative to ambienttemperature curve 25, in order to compensate for time drift of said RCMgenerated curves 26, 27, 27 a, 28, and 28 a relative to said ambienttemperature curve 25. Since SCM compensation compensates for time drift,which is generally small compared to other forms of component drift(e.g., thermal offset drift), over a typical period (for example, a fewdays) between consecutive uses of the embodiment of FIG. 1, said SCMcompensation need not be performed frequently. Additionally, it is anadvantage of the present invention that said RCM generated curves, whichvary relative to one another in a predominantly linear fashion overtime, consequently permit a single operation in SCM, at a singlearbitrary ambient temperature, which is sufficient to accurately adjustthe relative positions of said curves, with respect to ambienttemperature curve 25, across the temperature range of operation, for agiven session of operation. Said compensation for time drift of systemcomponents is performed to within an estimable accuracy, to which normaloperation will subsequently be limited. (See SCM steps 213 and 216,below.) The estimable accuracy, at any time during operation, is basedupon several factors, which it is also the function of SCM to calculate,including, and based upon:

a) elapsed time since the last operation in RCM, relating to non-linearcurve drift over time (i.e., change in curve “shape”), and laterreferred to as translation error (discussed in detail in connection withSCM step 213 in FIG. 12B);

b) method used to describe RCM generated curves, and the number ofpoints used to describe those curves (referred to above as interpolationerror); and

c) current warm-up status (based upon elapsed time since power-on).

SCM is preferably initiated only once, during each period of continuousoperation, preferably shortly after power is applied to the systemcomponents, permitting active components to warm up. Thermistors 3 and 4in FIG. 1 must be at substantially the same temperature during operationin SCM, in order to effect a measurement that can be compared to RCMmeasurements taken earlier, under the same circumstances ofsubstantially equal temperature between thermistors 3 and 4, asdescribed above. Preferably, thermistors 3 and 4 are normally in closeenough proximity to permit a mechanical assembly to place thermistors 3and 4 in good thermal contact, temporarily, during SCM (e.g., packedwithin a thermally conducting sleeve), so that said thermistorsexperience the same temperature, as described above. Alternatively, athermally homogeneous environment, such as a constant temperature bath,or even melting ice, can be used to help maintain thermistors 3 and 4 atsubstantially the same temperature during SCM. After operation in SCM,the RCM generated curves will have been repositioned, relative toambient temperature, thus compensating for time drift of said curves, sothat regular measurements of ambient temperature, during the operationalmode, using one of thermistors 3 or 4 subjected to ambient temperature(thermistor 3 in the preferred embodiment of FIG. 1), is sufficient todetermine drift-corrected ambient temperature, and thus permits accuratetemperature difference measurements (between thermistors 3 and 4), onthe order of micro-degrees centigrade, without the need for operating inSCM again during a continuous period of operation. Since operation inSCM is preferably conducted only once during a session of operation ofthe embodiment of FIG. 1, at a single, arbitrary ambient temperature,and practically instantaneously, SCM need not noticeably delay normalsystem operation (which subsequently continues without interruption),beyond considerations associated with holding thermistors 3 and 4 atsubstantially the same temperature during the brief period associatedwith SCM. A flow diagram, generally representing each of the individualsteps involved in operation of the preferred embodiment of the presentinvention in the standard calibration mode, is shown in FIG. 12B. SCMconsists of seven steps, described as follows.

SCM step 1 (210 in FIG. 12B): First, an ambient temperature measurementis acquired. Referring to FIG. 1, switch 14 is operated to connectambient temperature amplifier 13 to bridge node 11, resulting in anambient temperature measurement voltage from A/D converter 17, referredto hereafter as ambient temperature point 30, as shown in FIG. 3 (FIG. 3depicts an expanded portion of the lower left corner of the graph ofFIG. 2). Said ambient temperature point 30 is stored in memory 19, andis subsequently available for processing. For the purpose of thefollowing description of operation in SCM, said ambient temperaturepoint 30 is placed on ambient temperature curve 25, as shown, such thatsaid point's horizontal and vertical axis coordinates correspond to thesame measurement (in the present example, this measurement is 1.6RATI's, or sixteen LSB's, as shown in FIG. 3). Referring to FIG. 1, notethat reference node 16 is always connected to ambient temperatureamplifier 13, such that in this SCM step 210, the output of ambienttemperature amplifier 13 is the amplified difference between the voltageat bridge node 11 and the voltage at reference node 16.

SCM step 2 (211 in FIG. 12B): Next, a substantially time stable (incontrast to time and temperature stable) reference measurement isacquired for ambient temperature amplifier 13, that will contribute tocompensation for drift of the ambient temperature curve 25 (FIG. 3)relative to other RCM generated curves. Referring to FIG. 1, during SCMstep 211, switch 14 is operated to connect ambient temperature amplifier13 to reference node 15, so that the output of ambient temperatureamplifier 13 is the amplified difference between the voltage atreference node 15 and the voltage at reference node 16. The resultingreference measurement voltage, converted to digital form by A/Dconverter 17, hereafter referred to as ambient reference point 31 inFIG. 3, and more generally referred to as an actual ambient reference,is stored in memory 19.

Additionally, again referring to FIG. 3, for the purpose of thefollowing description of operation in SCM, said ambient reference point31 is placed directly on the vertical line 32, associated with ambienttemperature point 30, acquired above, during SCM step 210, as shown.Note that ambient reference point 31 is not situated directly on ambientreference curve 26, representing a time drift of said ambient referencecurve 26 relative to ambient temperature curve 25. If there were zerotime drift of components associated with the ambient temperaturemeasurement, then measured ambient reference point 31 would have beenplaced at the position of point 34 on ambient reference curve 26. Thedifference between points 31 and 34 will serve below to illustrate aninitial, estimated time drift adjustment, in SCM step 212, which islater made more exact during SCM step 213.

Also, referring to FIG. 1, in the following description it is assumedthat the ambient temperature, as detected by bridge node 11, issufficient to describe ambient temperature. However, it will beappreciated that ambient temperature, as detected by either, or both,bridge nodes 11 and 12, or even an additional temperature sensor, couldconceivably be used for this purpose. It will also be appreciated thatsaid additional temperature sensor could conceivably be connected inseries with a resistor, and said series-connected additional temperaturesensor and resistor connected in parallel with other series-connectedthermistor-resistor pairs (e.g., thermistor 4 and resistor 24) acrossmeasurement bridge voltage 21. It will also be appreciated by thoseskilled in the art that the use of an additional temperature sensor, andresistor, if said additional sensor is a thermistor, with substantiallythe same time drift characteristics as thermistors 3 and 4, could beused to acquire an additional thermal offset curve, over an ambienttemperature range, which conceivably could be used to provide additionaltime drift compensation for measurement thermistors 3 and 4.

SCM step 3 (212 in FIG. 12B): Referring again to the graph of FIG. 3,depicting an expanded view of the lower left portion of FIG. 2, recallthat ambient temperature point 30, acquired above during SCM step 210and placed on ambient temperature curve 25, was associated during SCMstep 211 with ambient reference point 31, as indicated by dashedconnecting line 32, which connects points 30, 31, and 34. In this SCMstep 212, the translation of RCM generated curves 25 and 26, due tocomponent time drift, is estimated. This translation is relative toother offset curves acquired during RCM, and following the estimation ofsaid translation in this SCM step 212, said translation is made moreexact, during SCM step 213.

Referring to FIG. 3, the above said translation will correctdiscrepancies, due to ambient temperature measurement time drift, incorrelations between ambient temperature points with points on other RCMgenerated curves (such as temperature difference offset curves, datapoints on which are associated with specific ambient temperature pointson ambient temperature curve 25, as described above). Saiddiscrepancies, due to time drift in components associated with theambient temperature measurement, if not compensated, would result in anincorrect ambient temperature measurement, and consequently, forexample, an incorrect correlation to expected zero temperaturedifference measurements associated with temperature difference curves 27and 27 a (FIG. 2).

Referring to FIG. 3, in order to effect the estimated translation ofambient temperature curves, relative to other RCM generated curves,points 30, 31, and 34 are used to initially estimate time drift ofcomponents associated with the ambient temperature measurement (e.g.,ambient temperature amplifier 13, A/D converter 17, thermistors 3 and 4,and bridge resistors). Recall that although to a lesser degree than inoffset drift, gain drift, over time, usually associated with resistorand thermistor time drift, is manifested in a predominantly lineartranslation of offset curves, relative to one another, in contrast to achange in curve “shape”. The estimated translation is accomplished byfirst determining, based upon the voltage level of ambient temperaturepoint 30, where one would expect the current ambient reference point 31to have been measured, if there were zero time drift of componentsassociated with the ambient temperature measurement since the last RCMoperation, i.e., point 34, generally referred to as an estimated ambientreference, on ambient temperature reference curve 26. The discrepancybetween point 34 and the measured voltage level of ambient referencepoint 31, acquired during SCM step 211, above, is used to estimate saidtime drift, as described below.

Recall that each discrete point generated during RCM, on any RCMgenerated curve, is associated in memory 19 (FIG. 1) with one point,referring to FIG. 3, on ambient temperature curve 25, representing themeasured ambient temperature at which said discrete point was acquired.Referring to FIG. 3, the voltage level associated with measured ambienttemperature point 30, provided by A/D converter 17 (FIG. 1), forindicating ambient temperature, is situated between known calibrationpoints 30 a and 30 b, both acquired during RCM (RCM step 201), onambient temperature curve 25, as shown. Note that both points 30 a and30 b were each associated during RCM, in memory 19, with specific points34 a and 34 b, respectively, on RCM generated ambient reference curve26, as depicted by dashed connecting lines 30′ and 30″, respectively.Note also that ambient temperature point 30 itself is not associatedwith a specific calibration point on RCM generated ambient referencecurve 26, because it is situated between calibration points, as shown.Therefore, in order to correlate ambient temperature point 30 on RCMgenerated ambient temperature curve 25 to a corresponding point on RCMgenerated ambient reference curve 26, as required by SCM step 212 todetermine where one would expect the current ambient reference point 31to be, if there were zero time drift of components associated with theambient temperature measurement since the last RCM operation (i.e., theexact position of point 34), it is necessary to determine the relativeposition of ambient temperature point 30, between known calibrationpoints 30 a and 30 b, on RCM generated ambient temperature curve 25. Inthe present example, ambient temperature point 30 is 60% of one RATIbetween calibration points 30 a and 30 b on RCM generated ambienttemperature curve 25, as shown. The fraction of 60% is hereafterreferred to as an interpolation fraction, and is used to estimate thepoint corresponding to current ambient temperature on RCM generatedambient reference curve 26 between known ambient reference calibrationpoints 34 a and 34 b, where one would expect the current actual ambientreference point 31 to be, if there were zero time drift of allcomponents associated with the ambient temperature measurement since thelast RCM operation, said estimated ambient reference point representedin FIG. 3 as point 34. The difference between said actual ambientreference point 31 and said estimated ambient reference point 34,considered at this stage to approximate the time drift associated withcomponents associated with ambient temperature measurement detected viabridge node 11, is identified in FIG. 3 as component time drift (CTD)33.

Note that in FIG. 3 calibration points are spaced relatively far apart,in order to better illustrate the method for drift compensation. It willbe appreciated by those skilled in the art that in practice, asmentioned briefly above, the embodiment of the present invention willstrive to acquire a sufficient number of calibration points, during RCM,such that interpolation error is minimized in all measurements.

Referring to FIG. 1, note also that low time drift, metal film referenceresistor 5 and wirewound potentiometer 6 of reference bridge 1,generating ambient reference potentials 15 and 16, are maintained nearthe common mode voltage of ambient temperature amplifier 13, in order tominimize the effect of possible time drift of amplifier common modeerror for the common mode range of input voltages, potentially seen byambient temperature amplifier 13, when connected to bridge node 11. Thatis, the possible time drift of common mode error, which may varyaccording to the common mode voltage seen by ambient temperatureamplifier 13, is minimized by adjusting the resistance values ofresistors 5 and/or 6, such that the voltage level at node 15 is as closeas practicable to voltage levels of bridge node 11, over the expectedoperating temperature range of the embodiment of FIG. 1. Additionally,as mentioned above, this is preferably done such that the gain ofambient temperature amplifier 13 can be maximized, without clipping thedifferential signal from ambient temperature amplifier 13, representingthe amplified voltage difference between nodes 11 and 16, and so thatneither the gain of ambient temperature amplifier 13, nor the voltageassociated with reference node 16, need be variable. Nevertheless, thoseskilled in the art will appreciate that such solutions as variable gainare conceivable, and could be implemented economically. However, asnoted above, though not apparent from thermal offset curves, as depictedin FIGS. 2-8 and 10-11, over any given ambient temperature range, zerodifference temperature (and other thermal offset curve) measurements ofthe preferred embodiment of FIG. 1 typically vary two to three orders ofmagnitude more slowly than ambient temperature measurements. Thispermits the gain of ambient temperature amplifier 13 to be two to threeorders of magnitude lower than the gain of difference temperatureamplifiers 7 and 7 a, in order to achieve a given temperature differencemeasurement resolution.

Additionally, referring to FIG. 3, recall that CTD 33 currently onlyprovides an estimate of component time drift associated with the ambienttemperature measurement. CDT 33 is preferably made more accurate belowduring SCM step 213.

SCM step 4 (213 in FIG. 12B): Referring to FIG. 3, in this SCM step 213,RCM generated ambient temperature curve 25 and RCM generated ambientreference curve 26 are translated relative to other RCM generatedcurves, in order to compensate for error due to ambient temperaturemeasurement time drift (estimated above during SCM step 212 as CTD 33).This is accomplished in this SCM step 213 by making CTD 33 more exact,and then executing the translations of ambient temperature curve 25 andambient reference curve 26, accordingly, relative to other RCM generatedcurves, so that expected measured values on said other RCM generatedcurves can accurately correspond to measured ambient temperature valuesat the time SCM is performed, and shortly thereafter (during theoperational mode), such that time drift of said other RCM generatedcurves, relative to measured ambient temperature, is substantiallycompensated. Additionally, the use and derivation of translation error(error associated with the translation of curves), is later described.

Referring to FIG. 4, depicting the measurements last acquired, above,during SCM step 212 in connection with FIG. 3, ambient temperature curve25 and ambient reference curve 26 are first translated vertically, inthe following analysis, by the amount initially approximated above byCTD 33. This vertical translation serves as an initial approximation ofambient temperature measurement time drift, that will later facilitate amore exact calculation of the extent to which RCM generated curves 27,27 a, 28, and 28 a (FIG. 2) are horizontally translated, relative to RCMgenerated ambient temperature curve 25, so that subsequent ambienttemperature measurements will be corrected for said time drift.

Referring to FIG. 4, RCM generated curves 25 and 26 are first translatedvertically, by the amount CTD 33, determined above during SCM step 212.The new curves are identified in FIG. 4 as SCM translated ambienttemperature curve 41 a and SCM translated ambient reference curve 42 a,respectively. This vertical translation will facilitate an approximationas to the horizontal translation of ambient temperature curve 25 andambient reference curve 26, required to compensate for time drift ofactive and passive components associated with ambient temperaturemeasurement.

Next, again referring to FIG. 4, the positions of ambient temperaturepoint 30 and ambient reference point 31 are horizontally translated byamount horizontal translation (HT) 35, more generally referred to as anambient signal offset, such that they are re-positioned close to newlytranslated curves 41 a and 42 a, respectively. The horizontaltranslation is a direct consequence of vertical translation of ambienttemperature curve 25 and ambient reference curve 26 by amount CTD 33, asdescribed above, and is conducted such that the values of ambienttemperature point 30 and ambient reference point 31, as measured on thevertical axis, remain constant throughout said horizontal translation,i.e., reflecting the fact that their measured (Y-axis) values arepreserved throughout the translation. Additionally, the positions ofambient temperature point 30 and ambient reference point 31 retain equalX-axis coordinates throughout the horizontal translation, as shown,reflecting the fact that they continue to represent measurements at asingle, equal ambient temperature throughout the translation. The new,translated points formerly associated with points 30 and 31 areidentified as translated ambient temperature point 43 and translatedambient reference point 44, respectively, after HT 35, as shown in FIG.4. Note that HT 35, applied during this SCM step 213, is anapproximation that can be useful to initially localize the currentambient temperature on offset curves that can conceivably, in thegeneral case, be high order functions of ambient temperature.Consequently, it is useful to initially localize the current ambienttemperature on the graph of FIG. 4, as described above, in contrast toskipping the initial localization and simply applying a function totranslate RCM generated curves 25 and 26 to “fit” the measured ambienttemperature and ambient reference points 30 and 31, respectively.

Referring to FIG. 4, it should also be noted that initially approximatedHT 35, of ambient temperature curve 25, would have the following effecton subsequent measurements (HT 35 is made more exact later). Prior tohorizontal translation, HT 35, with ambient temperature point 30 beingassociated with horizontal axis measurement 37, the expected differencemeasurement between bridge nodes 11 and 12 with thermistors 3 and 4 atthe same temperature would be associated with point 39 in FIG. 4, asshown on the first difference temperature curve 27 from differencetemperature amplifier 7. Also, the expected difference referencemeasurement would be associated with point 40 in FIG. 4, as shown on thefirst difference reference curve 28 from difference temperatureamplifier 7. Additionally, the expected ambient reference measurementwould be associated with point 34 in FIG. 4, as shown on ambientreference curve 26 from ambient temperature amplifier 13.

Referring to FIG. 4, following the application of HT 35, resulting fromvertical translation of ambient temperature curve 25 and ambientreference curve 26 by amount CTD 33, if the same ambient temperatureassociated with horizontal axis point 37 is measured, now correspondingto ambient temperature point 43 on SCM translated ambient temperaturecurve 41 a, then the expected zero difference temperature measurementbetween bridge nodes 11 and 12 with thermistors 3 and 4 at the sameambient temperature would be 39′ on the first difference temperaturecurve 27 from difference temperature amplifier 7. Also, the expecteddifference reference measurement would be 40′ on the first differencereference curve 28 from difference temperature amplifier 7.Additionally, the expected ambient reference measurement would be 44, asshown. However, note that point 44 is not precisely situated ontranslated ambient reference curve 42 a. This reflects a discrepancy,that is resolved below, with methods for determining HT (so farapproximated by HT 35) more precisely, so that points 30 and 31 aretranslated to both be situated substantially on translated ambienttemperature and translated ambient reference curves (so far approximatedby curves 41 a and 42 a, respectively).

Referring to FIG. 4, one way to determine the above verticaltranslations of ambient temperature curve 25 and ambient reference curve26 associated with SCM step 213, such that points 30 and 31 can both betranslated horizontally to be situated more precisely on said verticallytranslated curves, is to employ a least squares curve fit to both RCMgenerated curves 25 and 26, such that a polynomial is created for eachof said curves. The creation of such a polynomial by the least squaresmethod is well known in the art, and said polynomials for RCM generatedcurves 25 and 26 will be of the form: $\begin{matrix}{Y_{ambtemp25} = {\sum\limits_{i = 0}^{I}{a_{i}( X_{25} )}^{i}}} & {{Equation}\quad 1} \\{Y_{ambref26} = {\sum\limits_{i = 0}^{N}{b_{i}( X_{26} )}^{i}}} & {{Equation}\quad 2}\end{matrix}$

where, referring to FIG. 4, (Y_(ambtemp25), X₂₅) is a point on ambienttemperature curve 25; (Y_(ambref26), X₂₆) is a point on ambientreference curve 26; a_(i) are polynomial coefficients corresponding toambient temperature curve 25; b_(i) are polynomial coefficientscorresponding to ambient reference curve 26; and N is the desired orderof the resulting polynomial. Note that the order of Equation 1 is one,indicating a straight line, as is appropriate for ambient temperaturecurve 25.

Referring to FIG. 5, with a vertical translation 6 (corresponding to amore precise estimate of CTD 33, determined above) applied to each ofthe RCM generated curves 25 and 26, in order to translate them to newSCM translated curves 41 b and 42 b shown in FIG. 5, respectively, sothat points 30 and 31 can be translated to both be situatedsubstantially upon said SCM translated curves 41 b and 42 b, the abovepolynomials become: $\begin{matrix}{Y_{SCMxlated41b} = {{\sum\limits_{i = 0}^{I}{a_{i}( X_{41b} )}^{i}} + \delta}} & {{Equation}\quad 3} \\{Y_{SCMxlated42b} = {{\sum\limits_{i = 0}^{N}{b_{i}( X_{42b} )}^{i}} + \delta}} & {{Equation}\quad 4}\end{matrix}$

where, (Y_(SCMxlated41b),X_(41b)) are points on newly translated SCMtranslated ambient temperature curve 41 b, and(Y_(SCMxlated42b),X_(42b)) are points on newly translated SCM translatedambient reference curve 42 b shown in FIG. 5. Note that coefficientsa_(i) and b_(i) remain the same as in Equations 1 and 2, indicating thatcurve shape is unchanged from that of RCM generated curves 25 and 26(error associated with this assumption is referred to as translationerror, and is discussed below, as part of SCM step 213). Note thatstarting from a given point (X₂₅, Y_(ambtemp25)), situated on RCMgenerated ambient temperature curve 25 (Equation 1) to be translatedsuch that its Y-axis value remains constant, when repositioned to besituated upon curve 41 b (via Equation 3, i.e., RCM generated ambienttemperature curve 25 vertically translated by amount δ), said givenpoint (Y_(ambtemp25),X₂₅) is translated to position(Y_(SCMxlated41b),X_(41b)), with Y_(ambtemp25) in Equation 1 set equalto Y_(SCMxlated41b) in Equation 3. Then, due to the effect of verticaltranslation 67 in Equation 3, the corresponding value for X₂₅ inEquation 1 is translated (horizontally) to X_(41b) Similarly, for agiven value Y_(ambtemp26) in Equation 2, due to the effect of verticaltranslation 67 in Equation 4, the corresponding value for X₂₆ inEquation 2 is translated (horizontally) to X_(42b) (Equation 4). Notealso that the horizontal translation (X_(42b)−X₂₆) is identical to(X_(41b)−X₂₅).

Referring to FIG. 5, in order to solve for the desired verticaltranslation δ 36 a, affecting both RCM generated curves 25 and 26, suchthat ambient temperature point 30 and ambient reference point 31 can bemore precisely translated horizontally to be each positionedsubstantially on SCM translated curves 41 b and 42 b, respectively,values in Equation 3 and Equation 4 are set as follows. Y_(SCMxlated41b)is the measured voltage associated with ambient temperature point 30,and Y_(SCMxlated42b) is the measured voltage associated with ambientreference point 31 (reflecting, as mentioned above, that the verticalaxis values of ambient temperature point 30 and ambient reference point31 remain constant throughout the horizontal translation, i.e.,preserving their measured values); and X_(41b) is assumed to equalX_(42b) (as mentioned above), such that the positions of ambienttemperature point 30 and ambient reference point 31 retain equal X-axiscoordinates throughout said horizontal translation, reflecting the factthat they represent measurements conducted at a single, equal ambienttemperature.

Referring to Equations 3 and 4, with the values for Y_(SCMxlated41b) andY_(SCMxlated42) b known, and the values for coefficients a_(i) and b_(i)also known (from the polynomial curve fit of Equations 1 and 2, above),it remains to solve for δ 36 a (FIG. 5) and X_(41b) (which equalsX_(42b)). Hence, Equations 3 and 4 are a pair of nonlinear equations,with a pair of unknown variables, which can therefore be solved bymethods well known to persons skilled in the art. Referring to FIG. 5,the value for δ 36 a is preferably determined first, followed by adetermination of X_(41b) (which equals X_(42b)). Once X_(41b) andX_(42b) are determined, they are subtracted from X_(25b) (which equalsX_(26b) and corresponds to the X-axis value of points 30 and 31),resulting in the appropriate final horizontal translation (final HT) 45,more generally referred to as a final ambient signal offset, required totranslate ambient temperature point 30 and to translate ambientreference point 31 to be substantially situated on the newly translatedcurves 41 b and 42 b, respectively, formerly corresponding to RCMgenerated ambient temperature curve 25 and ambient reference curve 26,respectively.

A less general, and simpler approach, with comparable accuracy, andpreferred when a sufficient number of calibration points is acquiredduring RCM, is considerably less computationally intensive. Referring toFIGS. 4 and 5, the simpler approach is directed toward making efficientuse of the discrete nature of measurements with the embodiment of FIG.1, to solve the problem of translating RCM generated ambient temperaturecurve and RCM generated ambient reference curve 26, described above,such that time drift is accurately reflected in said translation, i.e.,such that ambient temperature point 30 and ambient reference point 31can be horizontally translated, so that said points are substantiallysituated on the resulting translated curves, as described above.

Referring to FIG. 5, the less computationally intensive approach is asfollows. First, determine the exact region of RCM generated curves 25and 26, where vertical translation of the RCM generated curves 25 and 26is likely to occur. This is performed by translating both RCM generatedambient temperature curve 25 and RCM generated ambient reference curve26 by an amount initially approximated by CTD 33 as done above, and asshown in FIGS. 4 and 5. These initially translated curves are identifiedin FIGS. 4 and 5 as initially translated ambient temperature curve 41 aand initially translated ambient reference curve 42 a.

Next, referring to FIG. 5, choose the closest RCM calibration points tothe Y-axis value of ambient temperature point 30 and ambient referencepoint 31 on initially translated curves 41 a and 42 a, respectively.Referring to FIG. 5, the closest calibration points, on initiallytranslated curve 41 a, to the Y-axis value of ambient temperature point30 are identified as 43 a and 43 b. The closest calibration points, oninitially translated curve 42 a, to the Y-axis value of ambientreference point 31 are identified as 44 a and 44 b.

Next, again referring to FIG. 5, form two linear equations, one of saidlinear equations representing the line between calibration points 43 aand 43 b on initially translated ambient temperature curve 41 a, and theother of said linear equations representing the line between calibrationpoints 44 a and 44 b on initially translated ambient reference curve 42a. Said two linear equations are of the form:

Y_(41a)=A_(41a)X_(41a)+B_(41a)  Equation 5

Y_(42a)=A_(42a)X_(42a)+B_(42a)  Equation 6

where, referring to FIG. 5, Equation 5 represents the line betweenpoints 43 a and 43 b, on initially translated ambient temperature curve41 a, and Equation 6 represents the line between points 44 a and 44 b,on initially translated ambient reference curve 42 a. In Equation 5, thevalues for coefficients A_(41a) and B_(41a) may be determined bysubstituting the X and Y values for the pair of points 43 a and 43 b,and solving the resulting pair of equations for A_(41a) and B_(41a), asis well known to persons skilled in the art. The same approach, usingpoints 44 a and 44 b, is used to determine the values for A_(42a) andB_(42a) in Equation 6.

Next, referring to FIG. 5, an additional vertical translation δ 36 isapplied to the initially translated curves 41 a and 42 a (initiallytranslated vertically by the amount identified as CTD 33), so that saidinitially translated curves 41 a and 42 a are finally translatedvertically by the total amount δ 36 a (δ=δ′+CTD) to final positionsrepresented by SCM translated ambient temperature curve 41 b and SCMtranslated ambient reference curve 42 b, respectively. Said additionaltranslation is effected such that ambient temperature point 30 andambient reference point 31 can be horizontally translated tosubstantially fit on said SCM translated ambient temperature curve 41 b(at point 43′) and SCM translated ambient reference curve 42 b (at point44′), respectively. In order to determine the value for δ′ 36, the aboveEquations 5 and 6, representing initially translated ambient temperaturecurve 41 a and initially translated ambient reference curve 42 a,respectively, are modified to include said additional verticaltranslation δ′ 36.

Y_(41a)=A_(41a)X_(41a)+B_(41a)+δ′  Equation 7

Y_(42a)=A_(42a)X_(42a)+B_(42a)+δ′  Equation 8

Then, referring to FIG. 5, with the values for A_(41a), B_(41a),A_(42a), and B_(42a) determined, as described above, and substitutingthe Y-axis value (measured voltage level) of ambient temperature point30 for Y_(41a) and substituting the Y-axis value (measured voltagelevel) of ambient temperature point 31 for Y_(42a), and assuming thatX_(41a)=X_(42a) (reflecting the fact that substituted points 30 and 31share a single measured ambient temperature value, and will continue todo so, throughout the horizontal translation, which finally positionsthem at point 43′ and point 44′, respectively), the result is a pair ofequations with a pair of unknown variables, namely, δ′ and X_(41a)(which equals X_(42a)). This pair of equations is then solved for thepair of unknowns, δ′ and X_(41a) (which equals X_(42a)), by methods wellknown to persons skilled in the art. Specifically, solving for δ′yields: $\begin{matrix}{\delta^{\prime} = {\lbrack \frac{( {A_{41a}*A_{42a}} )}{( {A_{42a} - A_{41a}} )} \rbrack*\lbrack {\frac{( {Y_{41a} - B_{41a}} )}{A_{41a}} - \frac{( {Y_{42a} - B_{42a}} )}{A_{41a}}} \rbrack}} & {{Equation}\quad 9}\end{matrix}$

Referring to FIG. 5, once δ′ 36 is determined, X_(41a) (which equalsX_(42a)) is calculated, using either Equation 7 or Equation 8, above,and substituting, respectively, the Y-axis value (measured voltagelevel) of ambient temperature point 30 for Y_(41a) (Equation 7) orsubstituting the Y-axis value (measured voltage level) of ambienttemperature point 31 for Y_(42a) (Equation 8), respectively. OnceX_(41a), corresponding in the present example to the X-axis value ofpoint 43′ is known, it is subtracted from X₂₅, corresponding in thepresent example to the X-axis value of point 30, to yield the finalvalue of horizontal translation, final HT 45, necessary to moreprecisely reposition ambient temperature point 30 on SCM translatedambient temperature curve 41 b (at point 43′) and to simultaneouslyreposition ambient temperature point 31 on SCM translated ambientreference curve 42 b (at point 44′). It will be appreciated that anidentical process, subtracting X_(42a) (X-axis value of point 44′) fromX₂₆ (X-axis value of point 31) will arrive at the same value for finalHT 45. The value of final HT 45 in the present example is five LSB's ofA/D converter 17 (FIG. 1), or 0.5 RATI, as shown in FIG. 5. Note thevalue of final HT 45 in comparison to the initial HT 35 that wasdetermined in connection with FIG. 4, and as a result of using aninitial vertical translation of CTD 33 as an approximation, shown inFIG. 5.

Note also the dependence on relatively consistent RCM curve contour, or“shape”, over time, which permits these relative translations of anentire curve from one location to another based upon measurements at asingle ambient temperature. Inaccuracies resulting from the assumptionof consistent curve “shape”, over time, associated with a given curve,are hereafter referred to as translation error for said given curve.Translation error has an effect on the accuracy of translationsdescribed above, as well as on subsequent calculations, that depends onthose translations. For example, referring to FIG. 5, calculations whichdetermine final HT 45, which are used to translate ambient temperaturecurve 25, so that it correctly compensates for time drift in ambienttemperature measurements, relative to other RCM generated curves, areaffected by translation error. The cause of translation error (TE) isdiscussed below, as part of the description of SCM step 213, along withits derivation and application to measurements in accordance with thepresent invention, since TE is a byproduct of the translations performedin SCM step 213.

Translation error (TE) is primarily the result of amplifier gain driftover time (e.g., resulting from feedback resistor drift over time),thermistor drift over time, and time drift of bridge resistors.Consequently, TE can be estimated, at the time of manufacture, basedupon combined component time drift specifications. In accordance withthe present invention, TE of a given curve can be regarded as a tendencyof the curve to change its “shape” as a function of time, in contrast toany uniform, linear translations of said curve (e.g., due to componentdrift, that is uniform throughout the temperature range of interest). Ifcurves are described as polynomials, TE is the tendency of all but thelinear coefficients of said polynomials to vary over time, that is,irrespective of any linear drift that may occur due to thermal andtemporal component drift. TE for any given RCM generated curve is then afunction of time since the last operation of the system components inRCM (at which time, the shapes of said curves were stored in memory 19in the form of discrete points). Since TE may also vary along thetemperature range of an RCM generated curve, it is conceivable that TEcan be quantified as a function of both time (elapsed since the lastoperation in RCM) and ambient temperature, as measured by ambienttemperature amplifier 13 (FIG. 1).

It should be noted that the method of the present invention compensatesfor any component time drift which manifests itself, in whole or inpart, as a linear translation in the thermal offset curves acquiredduring RCM. That is, while certain types of component time drift, suchas those associated with feedback resistors and thermistors, may to someextent be manifested as a change in “shape” of a given thermal offsetcurve over time, such drift is to a significantly greater extentmanifested as a linear translation of said given curve over time, andconsequently, is substantially compensated by the method of the presentinvention.

Referring to FIG. 6, as an example to illustrate the effect of TE,consider that at a time T, following the generation of RCM generatedcurves 25 and 26, during an operation of the embodiment of FIG. 1 inSCM, said RCM generated curves 25 and 26 undergo a linear, verticalshift by the process described above in SCM steps 210-213 (said shiftindicated in FIGS. 5 and 6 as δ 36 a), to become translated curves 41 band 42 b, respectively. However, referring to FIG. 6, if the shape oftranslated ambient reference curve 42 b has changed, to some extent, inthe intervening time T, since the last operation in RCM, in the formindicated by shaded region 52 (exaggerated for the sake ofillustration), in the vicinity of ambient reference measurement 31, andrepresented in FIG. 6 as part of curve 42 d, then the magnitude of thishypothetical change of shape, identified as translation error (TE) 52 ain FIG. 6, will have an effect upon the accuracy of translationsassociated with SCM steps 212 and 213. As can be seen from FIG. 6,curves 42 b and 42 d share the same value at the ambient temperature (online 32), at which ambient temperature point 30 and ambient referencepoint 31 are depicted. Consequently, at the ambient temperatureassociated with line 32, previously determined vertical and horizontaltranslations δ 36 a and final HT 45, respectively, will correctlycompensate for time drift of curves associated with the ambienttemperature measurement, only at said ambient temperature associatedwith line 32. Hence, though point 31 is measured correctly, under theinfluence of the change in curve shape 52, the use of point 31 todetermine the vertical and horizontal translations during SCM step 213would result in an incorrect determination of final HT 45, outside ofthe immediate vicinity of the ambient temperature associated with line32. In order to translate RCM generated ambient temperature curve 25 andRCM generated ambient reference curve 26, such that a final HT value iscorrectly determined for subsequent operations of the preferredembodiment outside of the ambient temperatures affected by TE 52 a, thevertical translation should actually be 36 a′=δ36 a+TE 52 a, as shown inFIG. 6. With the adjusted translation 36 a′, RCM generated ambienttemperature curve 25 is translated to curve 41 d, and RCM generatedambient reference curve 26 is translated to curve 42 d, and the modifiedvalue HT becomes 45′, as shown, in contrast to the earlier derived finalHT 45.

Referring to FIG. 6, note also that TE 52 a can be applied to translatedambient temperature curve 41 b, as well as to translated ambientreference curve 42 b, to illustrate TE. That is, the magnitude of TE 52a observed in the present example, as associated with translated ambientreference curve 42 b, may alternatively be applied directly to SCMtranslated ambient temperature curve 41 b, with the same effectivemodification on the resulting final HT 45, as can be seen by inspectionin FIG. 6.

As briefly mentioned above, TE is largely a function of component timedrift, particularly associated with feedback resistors and bridgeresistors, as well as thermistors. The effects of TE can be greatlyreduced through the use of time stable resistive components in thepreferred embodiment, such as standard metal film resistors,manufactured by Dale Electronics, of Norfolk, Nebr., which, as describedabove, offer stability over time comparable to much more expensivetemperature stable, and time stable, bulk metal foil and wirewoundresistors. Referring to FIG. 1, this preference for time stability inresistive elements, which serves to reduce TE, can be applied toresistive components of the preferred embodiment, excluding thermistors,and applies particularly to amplifier feedback resistors, referenceresistors 5 and 6, as well as to bridge resistors 23 and 24. Note thattime stability can also be substantially satisfied by the use ofwirewound potentiometers, which for convenience are used for resistors 6and 23 in the preferred embodiment of FIG. 1. However, it will beappreciated that these may be replaced by fixed metal film resistors, ifjustified by cost considerations. Derivations for TE, as a function oftime (elapsed since the latest operation in RCM) are preferablyinitially specified by manufacturers (e.g., as resistor time drift) ofindividual system components and subsequently combined to estimate TEassociated with all relevant components for a given RCM generated curve.Such an initial specification, however, applies to a range of componentsand temperatures, rather than specific parts and temperatures, andtherefore is only useful in estimating the maximum TE for similarcomponents, from the same manufacturer, over a specified temperaturerange.

An alternative, or addition, to the use of time stable resistivecomponents, for reducing the effects of TE, is to periodicallyre-acquire, i.e., update, RCM generated curves, by operating theembodiment of FIG. 1 in RCM over an available temperature range, asdescribed during the description of RCM, above. As described above,passive component drift, resulting in, for example, gain drift, ismanifested to a much greater extent as a linear translation in thermaloffset curves, than as a non-linear change, i.e., a change in curve“shape” over time, and consequently is substantially compensated by themethod of the present invention. However, such changes in curve “shape”will eventually affect the accuracy of measurements. The extent to whichthe effects of non-linear drift over a given time are compensatedcorresponds to a maximum time period, within which re-acquisition ofthermal offset curves is required, in order to achieve a givenmeasurement accuracy. This maximum period depends substantially on thetime stability (not the temperature stability) of passive components,such as gain feedback resistors. In order to determine the maximumperiod, between re-acquisition of offset curves, required to achieve agiven accuracy, the operation in RCM is preferably performed at the timeof manufacture, utilizing a constant temperature bath, capable ofproviding at least two known, repeatable temperatures (the reason for aconstant temperature bath relates to a process described below forempirically quantifying TE at the time of manufacture). Nevertheless,the method of the present invention permits a re-acquisition of thermaloffset curves over an arbitrary temperature range as described above, sothat subsequent re-acquisitions of thermal offset curves can beperformed, at any time, by the end user, without such costly calibrationequipment.

It will be appreciated by those skilled in the art that updated RCMcurves possess endpoints, representing either end of the “updated”calibrated ambient temperature range. It is conceivable that if a givencurve is sufficiently smooth, the endpoints can be extended byextrapolating beyond said endpoints, using curve extrapolation methodsknown to persons skilled in the art, and/or by noting characteristics ofpreviously acquired versions of said curve, beyond the range of saidendpoints. It is also conceivable that during normal operation, thesystem can warn the operator, or simply stop functioning, when ambienttemperature exceeds the limits of a calibrated range.

Alternatively, or as a means of more tightly defining manufacturersupplied specifications for TE, TE may be determined empirically, byusing additional operations in RCM, at different times, and comparingthe shapes of the resulting versions of RCM generated curves at saiddifferent times, using curve comparison techniques known to thoseskilled in the art. Note that the additional operations are conductedseparately from SCM step 213. However, these additional operations usedto empirically quantify TE are described below, since TE is a byproductof the translations performed during SCM step 213 and has a directimpact upon the accuracy of measurements conducted in this SCM step 213(such as final HT 45).

Generally, in order to determine TE for a given RCM generated curve, itis necessary to generate at least two versions of said curve, each at adifferent time. Next, any linear drift component of the two saidversions of said curve, generated at two different times, is quantified(e.g., by performing a linear curve fit algorithm, well known to personsskilled in the art), so that said two curves can be more easily comparedfor the effects of purely non-linear drift (i.e., TE). Then, pairs ofdiscrete points, each said pair associated with a particular ambienttemperature, are differenced, with said differences associated with theambient temperature measurement at which said pair was acquired, andadditionally associated with the time elapsed since the last operationin RCM. Thus, TE for a given curve can be defined as a function ofambient temperature measurement and elapsed time since the most recentoperation in RCM.

More specifically, a preferred approach that can be used to determine TEfor a given RCM generated curve, in the embodiment of FIG. 1, has theeffect of positioning two versions of a given RCM generated curve,acquired at two different times, such that they can be compared forvariations in curve “shape” (i.e., non-linear drift), and requires thatthe preferred embodiment of FIG. 1 be operated between two temperatures,at least one of which is repeatable. In accordance with this preferredapproach, referring to FIG. 7, an RCM generated curve 55 a is generatedduring an operation in RCM at a time T₀. Later, at a time T₁, anotherRCM generated curve 55 b is generated. These two operations in RCM areeffected between two temperatures, at least one of which is a repeatabletemperature, such as that of melting ice, so that the two operations inRCM can be performed by the end user, without expensive calibrationequipment. However, in the preferred embodiment, the two operations inRCM are effected at the time of manufacture, in order to establish TEfor the life of the system, such that both of said two operations in RCMare performed between two known, repeatable ambient temperatures, forexample, by placing the embodiment of FIG. 1 during RCM in a constanttemperature bath, that is ramped between the two known, repeatabletemperatures. As is known in the art, electronic component time driftdecreases over the life of components, so that TE, thus determined atthe time of manufacture, will provide a worst case TE estimate, for thelife of a system, comprising said components.

Referring to FIG. 7, operation in RCM between the two known, repeatabletemperatures at time T₀ results in a curve 55 a, with endpoints atmeasured ambient temperatures 56 a′ and 56 b′. A subsequent operation inRCM between the same two known, repeatable temperatures at a later timeT₁ results in another curve 55 b with endpoints at measured ambienttemperatures 56 a″ and 56 b″. Curves 55 a and 55 b are translateddiagonally with respect to each other, as shown in FIG. 7, representinga combination of linear and non-linear time drift of said curves over atime (T₁-T₀), which is exaggerated for the sake of illustration, andalso includes an exaggerated change in curve shape, identified as 52 inFIG. 6. By this method, the curves 55 a and 55 b, thus generated on theabove two occasions, are effectively positioned, for comparison of pairsof endpoints on curves 55 a and 55 b, with said pairs of endpoints, usedin said comparison, acquired at the same ambient temperatures. Two pairsof endpoints are indicated by connecting lines 56 c′ and 56 c″ in FIG.7, located at the endpoints of the temperature range, described above,and associated with the two known, repeatable temperatures, betweenwhich curves 55 a and 55 b were acquired. Connecting line 56 c′ connectsleft endpoints 55 a′ and 55 b′, and connecting line 56 c″ connects rightendpoints 55 a″ and 55 b″. The length and slope of lines 56 c′ and 56 c″can conceivably differ between the two known, repeatable temperatures,between which curves 55 a and 55 b were acquired, as shown in FIG. 7.Therefore, the length and slope of intervening connecting lines 56 d′and 56 e′, shown in FIG. 7, are preferably determined by linearlyinterpolating between the length and slope of endpoint connecting lines56 c′ and 56 c″. Thus, respective connecting lines 56 c′ and 56 c″,which connect points on curves 55 a and 55 b, that are acquired betweenthe same known ambient temperatures (measured as 56 a′ through 56 b′ and56 a″ through 56 b″, respectively) are used to estimate the length andslope of connecting lines 56 d′ and 56 e′, intended to connectintervening points on curves 55 a and 55 b, said intervening pointsacquired at the same ambient temperatures.

Referring to FIG. 7, the above said diagonal translation, which over anintervening time (T₁−T₀) modifies and translates curve 55 a to bere-positioned as curve 55 b, as shown, can be used to quantify TE acrossthe above measured ambient temperature range. First, a determination ismade of the maximum and minimum difference between each pair ofassociated points on RCM curves 55 a and 55 b, said associated pointsconnected by connecting lines 56 c′, 56 d′, 56 e′, and 56 c″ in FIG. 7.That is, each pair of calibration points, such as pair of points 55 a′and 55 b′, connected by connecting line 56 c′; pair of points 57 a and57 b, connected by connecting line 56 d′; pair of points 57 c and 57 d,connected by connecting line 56 e′; and pair of points 55 a″ and 55 b″,connected by connecting line 56 c″, are differenced, and saiddifferences are compared. The maximum difference among the set ofassociated pairs of points, hereafter referred to as Δmax, is identifiedas 58 in FIG. 7, and the minimum said difference among said pairs ofpoints is hereafter referred to as Δmin and is identified as 59 in FIG.7. Therefore, the maximum TE associated with the curves (acquired at twodifferent times) represented by curves 55 a and 55 b in FIG. 7, over themeasured range of temperatures, can be estimated as Δmax−Δmin. The useof this difference, Δmax−Δmin, in estimating TE eliminates the effect ofany linear shift between RCM curves 55 a and 55 b within the measuredrange of ambient temperatures (which is compensated by the method of thepresent invention), leaving only that portion of component drift that isnot manifested as a linear translation and consequently not directlycompensated by the method of the present invention. Since the elapsedtime T₁−T₀, between operations in RCM, is known (said elapsed time beingdetermined by timer 18 in FIG. 1), TE as a function of time t, for anRCM generated curve 55 (represented at times T₀ and T₁ as curves 55 aand 55 b, respectively, in FIG. 7) may be expressed asTE₅₅(t)=t*(Δmax−Δmin)/(T₁−T₀). Subsequent measurements within themeasured range of temperatures can be expected to exhibit TE₅₅(t), whereTE₅₅(t) is the TE associated with a said RCM generated curve 55, andwhere elapsed time t is the time elapsed since TE₅₅(t) was lastdetermined. Also, it will be appreciated that if one knows that theembodiment of FIG. 1 is operating in a specific, limited temperaturerange, the estimate for TE₅₅(t) can be improved by using only(Δmax−Δmin) within the specific limited temperature range for thecalculation of TE₅₅ (t)=t*(Δmax−Δmin)/(T₁−T₀). Additionally, it will beappreciated that if several ambient temperature ranges are thusdetermined, and associated with several functions TE(t), said severalfunctions can conceivably be associated with said several ambienttemperature ranges and consolidated into a single function of both timeand ambient temperature.

Referring to FIG. 7, it is also conceivable that a determination of TE₅₅(t) can be estimated without applying known repeatable temperatures toan operation in RCM, by identifying distinctive and unique slopecharacteristics, if they can be so identified, on various parts of RCMcurve 55 a acquired at time T₀, and substantially aligning them (e.g.,by horizontal translation) to the same said slope characteristics on RCMcurve 55 b acquired at time T₁. The identification of unique slopecharacteristics can be accomplished by applying a known curve fittingformula to RCM curves 55 a and 55 b, and, for example, taking thederivative of said formula at the points of interest to determine theslope at said points of interest. Next, points on RCM curves 55 a and 55b with uniquely matching patterns of slope characteristics can bealigned, by translating said RCM curves horizontally, relative to oneanother, so that the effect is that of a correspondence between pairs ofpoints, comparable to that shown for RCM curves 55 a and 55 b in FIG. 7,said pairs of points having substantially the same unique slopecharacteristics. Then, once again, the formula TE₅₅(t)=t *(Δmax−Δmin)/(T₁-T₀) can be applied, after determining (Δmax−Δmin), overthe temperature range of interest.

Yet another conceivable approach in determining TE, preferablyapplicable during a separate operation in SCM, involves taking an SCMmeasurement, as described above in connection with FIG. 5, to calculatevertical translation δ 36 a and final HT 45 at one or more particularpoints on a given RCM generated curve, relative to ambient temperaturecurve 25. In this case, said δ and final HT can then be used, referringto FIG. 7, to approximate a repositioning of an RCM generated curve 55 ato translated RCM generated curve 55 b, relative to ambient temperature,such that pairs of recorded points on both said RCM curves 55 a and 55 bcan be identified as being associated with substantially the sameambient temperature, the effect again being that of alignment shown forRCM curves 55 a and 55 b in FIG. 7. Then, once again, the formulaTE₅₅(t)=t * (Δmax−Δmin)/(T₁−T₀) can be applied, after determining(Δmax−Δmin), over the temperature range of interest.

The application of TE in the method of the present invention,specifically how it is used with other error terms to determine a finaldifference temperature error, is described in the discussion of theoperational mode, which follows the present discussion of SCM, which nowcontinues below.

SCM step 5 (214 in FIG. 12B): Referring back to FIG. 5, SCM step 214uses the SCM translated ambient temperature point 43′ to determine theposition 46 on RCM generated difference temperature curve 27, where onewould expect to find the current measured difference temperature point,if there were zero difference temperature between thermistors 3 and 4(FIG. 1), and if there were zero time drift of components associatedwith the difference temperature measurement since the last operation ofthe embodiment of FIG. 1 in RCM. Similarly, SCM step 214 determines theposition 49 on RCM generated difference reference curve 28 of the pointon said difference reference curve 28 where one would expect to find thecurrent difference reference point on difference reference curve 28, ifthere were zero time drift of components associated with differencetemperature amplifiers 7 (and 7 a) and AID converter 17, since the lastRCM operation, at which time said difference reference curves 28 and 28a were last stored (recall that during RCM, points on differencereference curves 28 and 28 a were acquired by using switches 10 and 10 ato connect the same bridge node 11 to both inputs of differencetemperature amplifiers 7 and 7 a). Thus, SCM begins to compensate fordrift of active and passive components associated with differencemeasurements.

Referring to FIG. 8, depicting the same portion of the graph of FIG. 2as shown in FIG. 5, translated ambient temperature point 43′, situatedon SCM translated ambient temperature curve 41 b, can be associated withthe corresponding point 46 on RCM generated first difference temperaturecurve 27 (measured via the first difference temperature amplifier 7 inFIG. 1). Similarly, referring to FIG. 2, note that the value of point 47on RCM generated second difference temperature curve 27 a (measured viathe second difference temperature amplifier 7 a), also associated withthe current ambient temperature, would correspond to a negative value.However, since both difference temperature amplifiers 7 and 7 a aresingle supply amplifiers in the preferred embodiment of FIG. 1, saidnegative value is amplified by said difference temperature amplifier 7 aas (essentially) a zero level voltage. Consequently, in the preferredembodiment of FIG. 1, employing a single ended supply voltage, only thevalue of corresponding point 46, referring to FIGS. 2 and 8, on RCMgenerated first difference temperature curve 27, hereafter referred toas temperature difference point 46, is used in the followingdescription. Nevertheless, referring back to FIG. 2, it will beappreciated that other points on RCM generated ambient temperature curve25, for example, ambient temperature point 75, will correspond to apositive valued point 76 on the second difference temperature curve 27a, and that, in general, the preferred embodiment of FIG. 1 mustdetermine which of the RCM generated difference temperature curves 27and 27 a to correlate to selected points of translated ambienttemperature curve 41 b in FIG. 8. Referring to FIG. 2, thisdetermination is preferably made by selecting the greater of the twovalues from the first and second RCM generated difference temperaturecurves 27 and 27 a associated with the current ambient temperature.Also, recall that in the preferred embodiment of FIG. 1, using a singleended supply voltage, thus requiring the use of two differencetemperature amplifiers 7 and 7 a, the possibility of simultaneousnegative values from both first and second RCM generated differencetemperature curves 27 and 27 a in FIG. 2, associated with a given pointof RCM ambient temperature curve 25, is effectively eliminated byadjusting measurement bridge resistor 23 (FIG. 1) accordingly, and/oradjusting offset voltages of difference temperature amplifiers 7 and 7 aaccordingly, using amplifier offset adjustment techniques well known topersons skilled in the art. Additionally, it will be appreciated bythose familiar with the art that the above discussion also applies tofirst and second RCM generated difference reference curves 28 and 28 a.Nevertheless, it is also conceivable that an embodiment may be poweredby a bipolar supply voltage, in which case only one differencetemperature amplifier is required, since said bipolar supply canaccommodate a single bipolar temperature difference amplifier,permitting negative difference voltage measurements, thus eliminatingthe need for the two difference temperature amplifiers 7 and 7 a.

Referring to FIG. 8, recall that the ambient temperature measurementpoint 43′ was subject to a final HT 45 of half (0.5) of a RATIcalibration point (or five LSB's of A/D converter 17 in FIG. 1), asdescribed above in SCM step 213. Additionally, recall that in memory 19,each discrete point of ambient temperature throughout the range of RCMgenerated ambient temperature curve 25, in addition to being associatedduring operation in RCM with a unique discrete point on RCM differencetemperature curves 27 and 27 a, is also associated during RCM operationwith a unique discrete point on RCM difference reference curves 28 and28 a. Also, recall that in SCM step 212, above, there was a linearinterpolation fraction (60%) related to the position of original ambienttemperature point 30 on RCM generated ambient temperature curve 25,between calibration points 30 a and 30 b. Note that this interpolationfraction changes to 10% when applied to translated ambient temperaturepoint 43′, between calibration points 43 a′ and 43 b′ on translatedambient temperature curve 41 b, as can be seen by inspection. Thisinterpolation fraction of 10%, associated with ambient temperature point43′, is now used to interpolate the position on RCM generated differencetemperature measurement curve 27 of the point on said differencetemperature measurement curve 27 between known temperature differencepoints 46 a and 46 b, where one would expect to find the currentmeasured difference temperature point 46, if there were zero time driftof components associated with the difference temperature measurement,since the last RCM operation. This estimated temperature differencepoint, hereafter referred to as estimated difference temperature (EDT),is represented in FIG. 8 as point 46.

Again referring to FIG. 8, a similar procedure is followed in order todetermine the point 49 on difference reference curve 28, which isassociated with ambient temperature measurement point 43′ situated onSCM translated ambient temperature curve 41 b. Again, recall that theambient temperature measurement point 43′ was subject to a final HT 45of 0.5 RATI calibration points (five LSB's), as described above in SCMstep 213. In the present example, associated with SCM translated ambienttemperature point 43′, the above mentioned interpolation fractionassociated with points 43 a′ and 43 b′ (10%) is now used to interpolatethe position on difference reference curve 28 between known differencereference points 49 a and 49 b, where one would expect to find thecurrent difference reference point on difference reference curve 28, ifthere were zero time drift of difference temperature amplifiers 7 (and 7a) and A/D converter 17, since the last RCM operation at which time saiddifference reference curves 28 and 28 a were last stored. This currentdifference reference point is represented in FIG. 8 as point 49 andhereafter referred to as estimated difference reference (EDR). Recallthat during RCM, points on difference reference curves 28 and 28 a wereacquired by using switches 10 and 10 a in FIG. 1 to connect the samebridge node 11 to both inputs of difference temperature amplifiers 7 and7 a.

SCM step 6 (215 in FIG. 12B): Next, referring to FIG. 1, the actualoffset of components which are associated with difference measurements,such as difference temperature amplifiers 7 and 7 a and A/D converter17, are determined, so that said actual offsets can be compared withexpected offsets, permitting component time drift associated withtemperature difference measurements to be estimated. Referring to FIG.1, the measurement of the said actual offset of components, which areassociated with difference measurements, are effected by operatingswitches 10 and 10 a to connect both difference amplifier inputs ofdifference temperature amplifiers 7 and 7 a to the voltage at bridgenode 11, such that all inputs of these amplifiers 7 and 7 a (i.e.,inputs 8, 9, 8 a, and 9 a) experience the same said voltage at bridgenode 11. Although time drift of common mode rejection ratio (CMRR) issubstantially compensated by the method of the present invention, thereason for using one of the bridge nodes in this measurement is tominimize the effects of any potential drift of amplifier CMRR over time,as described above in SCM step 212. Referring to FIG. 8, the differencebetween the resulting actual difference reference measurement (ADR) 50and the estimated difference reference EDR 49 (derived above during SCMstep 214) represents actual total drift of active components associatedwith difference measurement. The actual difference reference measurement(ADR) 50, subtracted from the value of the estimated differencereference (EDR) 49, equals a difference signal offset, more commonlyreferred to below as difference offset (DO) 51 in FIG. 8. Alternatively,as a way to determine the value for DO 51 in FIG. 8, it is conceivablethat the current value for DO 51 can be estimated by comparingpreviously stored values of DO on RCM difference reference curves 28 and28 a, each said DO value associated with an elapsed time since apreviously recorded DO value, and preferably associated with the currentpoint on the SCM translated ambient temperature curve 41 b,corresponding to current ambient temperature compensated for drift ofcomponents associated with the ambient temperature measurement. Then,using elapsed time since the last operation in RCM, as determined bytimer 18 in FIG. 1, the current DO value can be predicted. Additionally,it will be appreciated by those skilled in the art that said predictedvalues for DO, calculated at different measured ambient temperatures,may result in different values of DO associated with different ambienttemperatures. Nevertheless, as has been pointed out, due to thesubstantially linear drift of offset curves over time, a DO value cantypically be determined at a single arbitrary ambient temperature andused as a vertical translation for all points on RCM differencereference curve 28, over the entire ambient temperature range in whichsaid RCM difference reference curve 28 was acquired during operation inRCM, in order to substantially compensate said points on said RCMdifference reference curve 28 for time drift of components associatedwith difference temperature measurement. That is, in accordance with thepreferred embodiment, any point on said RCM difference reference curve28 is substantially compensated for time drift of components associatedwith difference temperature measurement by adding the value DO to saidpoint on said RCM difference reference curve 28. Referring to FIG. 8,the value DO 51 can thus be regarded as a vertical translation of theRCM difference reference curve 28, and if applied to all points on saidRCM difference reference curve 28, this translation would result in avertically translated difference reference curve, more generallyreferred to as a translated difference reference curve. It will beappreciated by those skilled in the art, however, that in theabove-mentioned case in which predicted values for difference signaloffset (or DO), calculated at different ambient temperatures, result indifferent values for DO at said different ambient temperatures, or inthe case in which data acquired during operation in SCM conducted atmore than one arbitrary ambient temperature otherwise results indifferent values for DO, which differ for each of the arbitrary ambienttemperatures, these different values of DO would result in a translateddifference reference curve that is translated by different amounts atdifferent ambient temperatures, depending on the value of DO associatedwith each said ambient temperature. It will be appreciated that in thiscase of multiple values for DO associated with different ambienttemperatures, values for DO, and the resulting required amount oftranslation of the difference reference curve (required to determinesaid translated difference reference curve), at ambient temperaturesother than those for which said multiple values of DO have beenassociated, may be estimated by methods of interpolation andextrapolation well known in the art. It will also be appreciated bythose skilled in the art that if, as described above for the preferredembodiment, DO 51 is a single value throughout the ambient temperaturerange over which RCM curves were acquired, or if DO is limited to anumber of discrete values over the said ambient temperature range (eachsaid discrete value associated with an ambient temperature) as notedabove in the case of predicted values for DO, then the additionalsoftware and memory overhead required to calculate a translateddifference reference curve from which to determine values for DO oversaid range of ambient temperatures may not be required, and instead onlysaid single value for DO, or said discrete values for DO, respectively,need to be determined and stored in memory in order to determine valuesfor DO that, with difference reference curve 28, could be used todetermine a translated difference reference curve. Rather than showingthe translated difference reference curve in the accompanying figures,which would unnecessarily complicate the figures, the value for DO 51 isassumed to be constant throughout the range of ambient temperatures overwhich offset curves were acquired during operation in RCM, in accordancewith the preferred embodiment of the present invention, and said valueDO 51 is used below to compensate individual points on RCM differencereference curve 28 for component drift over time. Additionally, as willbe described below in OM step 2 (221 in FIG. 12C), the value DO 51 isalso used, in accordance with the preferred embodiment of the presentinvention, for compensating points on the difference temperature curve27 for time drift, in order to compensate difference temperaturemeasurements for time drift of components associated with differencetemperature measurement.

It will be appreciated by those skilled in the art that DO 51, derivedabove without the use of measurement bridge components, consequentlyonly compensates for time drift of components associated (referring toFIG. 1) with A/D converter 17 and difference temperature amplifiers 7and 7 a, including associated feedback resistors. Referring to FIG. 1,the effect on difference measurements of different rates of time driftin measurement bridge resistances (thermistors 3 and 4 and resistors 23and 24) is substantially compensated by the fact that time drift inmeasurement bridge resistances substantially cancels itself in thebridge configuration of the preferred embodiment. With metal film andwirewound resistors, the value of differential time drift betweenresistors is already extremely low, so that the effects of saiddifferential resistance time drift between said resistances (relevant indifferential measurements) will not significantly impact differencetemperature measurements, with resolution on the order of micro-degreescentigrade. Note that thermistors generally possess a significantlyhigher time drift than other measurement bridge resistances, arepreferably matched by manufacturing them in close physical proximity toeach other, resulting in nearly identical time drift behavior.Additionally, it should be noted that any uncompensated time drift ofthermistors (or measurement bridge resistors), associated withdifferential temperature measurements, is manifested as TE associatedwith difference temperature curves 27 and 27 a, quantifiable asdescribed in SCM step 213 in connection with FIG. 7. Finally, anyuncompensated time drift of thermistors (or measurement bridgeresistors) associated with differential temperature measurement isadditionally manifested as difference time drift remainder (DTDR),quantifiable as described below, during SCM step 216 (in connection withFIG. 8), which, as described below, can be used to reduce the impact ofsuch errors manifested as said DTDR (including time drift of measurementbridge resistances) over multiple operations in SCM. It is also notedthat the time drift behavior of measurement bridge resistances(particularly thermistors) will have an effect on ambient temperaturemeasurements, but these are substantially compensated, as describedabove, by final HT 45 (FIG. 8) in connection with SCM step 213.

SCM step 7 (216 in FIG. 12B): Expected difference measurements are againcompared to actual difference measurements, this time for the purpose ofdetermining a difference temperature measurement error term to be usedduring the operational mode, when all error terms associated with themethod of the present invention are consolidated. Referring to FIG. 8,DO 51 is added to the value for the estimated difference temperature(EDT) point 46 (determined above during SCM step 214) on RCM temperaturedifference curve 27, in order to adjust EDT 46 for differencemeasurement component offset, represented by DO 51. The result is thepoint indicated by adjusted EDT (AEDT) 48. Next, referring to FIG. 1,switches 10 and 10 a are operated to re-connect inputs 8 and 9 ofdifference temperature amplifier 7 to bridge nodes 11 and 12,respectively, and to reconnect inputs 8 a and 9 a of differencetemperature amplifier 7 a to bridge nodes 12 and 11, respectively, inorder to measure the actual temperature difference value ATD 52 in FIG.8 (remember that during SCM, as in RCM, thermistors 3 and 4 are atsubstantially the same temperature).

Referring to FIG. 8, note the discrepancy between the estimateddifference value AEDT 48 (determined above) on the RCM generateddifference temperature curve 27 and the actual (measured) temperaturedifference value (ATD) represented by point 52. This discrepancy isidentified in FIG. 8 as difference time drift remainder (DTDR) 53. DTDR53 is an error term in all subsequent difference temperaturemeasurements. The application of the DTDR error term, as it applies tooperation of the embodiment of FIG. 1, is discussed in more detailbelow.

DTDR 53 is a cumulative error term associated with temperaturedifference measurements and is observed as a discrepancy, during SCMstep 216, between the predicted and actual difference temperaturemeasurements, AEDT 48 and ATD 52, respectively, in FIG. 8. DTDR 53 mayresult from unpredictable factors including RFI and sudden temperaturefluctuations, as well as from other, more predictable changes in thesystem, such as variations in the “shape” of RCM generated curves overtime at the current ambient temperature, i.e., TE (e.g., associated withuncompensated, differential time drift of measurement bridgeresistances); interpolation errors; and warm-up effects, related toelapsed time since power-on. Unpredictable environmental factors, suchas RFI, and sudden temperature variations may, to some extent, bereduced or eliminated by good design, or at least quantified for worstcase effect on the system. Warm-up error can also be essentially negatedif necessary, by requiring a warm-up period, or at least quantified forworst case effect on the system, based upon elapsed time since systempower was applied. Remaining DTDR error sources, particularly TE, changein an orderly fashion, compared to the above unpredictable factors.Consequently, over the course of operating the embodiment of the presentinvention in SCM, on several occasions, the effects of random sources oferror may, to some degree, be separated from the more ordered sources ofDTDR error, such as TE, e.g., by tracking DTDR, and noting consistentdeviations among successive DTDR measurements, taken during differentoperations in SCM. That is, it is conceivable that consistent deviationscan be quantified and, to some extent, subtracted from the total systemerror, evidenced by DTDR, thus increasing reportable system accuracy,each time the embodiment of the present invention is operated in SCM.For example, one of the ways this error reduction can be accomplished,if DTDR is tracked over N operations in SCM, over an ambient temperaturerange of, say, 1° C. , the result will be a series of N values for DTDR53 in FIG. 8, and N AEDT values 48 in FIG. 8, associated with saidspecific ambient temperature range of 1° C. If the mean value of the NDTDR values is greater than the standard deviation σAEDT, associatedwith said N AEDT values (and assuming that σDTDR<Δ(AEDT), then it may beassumed that an ordered, consistent shift in the relevant differencetemperature curve 27 (or 27 a) within the 1° C. temperature range hastaken place. Consequently, DTDR error may be reduced by an amount equal,or related to, the mean of the said N DTDR values, over the 1° C.temperature range. That is, a portion of said observed consistentbehavior of DTDR may be effectively subtracted from the DTDR error term,used later in a consolidated estimate of error terms, including DTDR, toquantify achievable system accuracy, since said observed consistentbehavior reflects a consistent shift in system behavior over multipleoperations in SCM. It will be appreciated by those skilled in the artthat said consistent behavior reflecting a consistent shift in systembehavior, such as in the shape of the difference temperature curve 27 ata given ambient temperature, may be used to modify the shape of thedifference temperature curve 27 at said given ambient temperature.Since, as will be described in connection with OM step 2 (221 in FIG.12C), DO 51 is used to compensate difference temperature measurements,it is conceivable that said modification in the shape of differencetemperature curve 27 at said given ambient temperature may be effectedby modifying the value for DO at said certain ambient temperature, whichwould also have the effect of modifying the shape of the translateddifference reference curve determined above in SCM step 215, since saidtranslated difference reference curve is determined by translatingdifference reference curve 28 by the value of DO.

Note that SCM step 211 and SCM step 215 are used to acquire referencemeasurements, which, referring to FIG. 1, require the use of switches10, 10 a, and 14. In the above description of the preferred embodiment,it facilitated explanation to sequence SCM steps in the above order.However, it is conceivable that SCM steps 211 and 215 may be conductedprior to the other SCM steps listed above, which would require fewerchanges in the state of switches 10, 10 a, and 14 in FIG. 1 duringoperation in SCM. Referring to FIGS. 1 and 9, it is also conceivablethat a mechanical switch, or push button, may be used to bothmechanically hold thermistors 3 and 4 in thermal contact during SCM, andat the same time operate switches 10, 10 a, and 14 (which may also bemechanical switches). For example, this could be accomplished by using amechanical “power on” switch that provides mechanically delayed movement(e.g., with a movement damping mechanism) after the operator presses the“power on” switch, with the delay permitting the system to substantiallywarm-up, while holding thermistors 3 and 4 in substantial thermalcontact, and while operating switches 10, 10 a, and 14 as required bySCM steps 211 and 215 for reference measurements, as described above.Subsequently, with thermistors 3 and 4 still in thermal contact, asrequired by operation in SCM, switches 10, 10 a, and 14 would bere-configured (responsive to the above mechanically delayed movement) tobe in a “measurement position” for difference measurements and ambienttemperature measurements, as described in the remaining SCM steps 210,212-214, and 216, which would be performed as described above (excludingSCM steps 211 and 215), beginning with SCM step 210. It will beappreciated by those skilled in the art, that the above mentionedmechanically delayed movements and switching actions could also beaccomplished by electronic means. Note that the above describedmeasurements in SCM are conducted quickly, and at a single ambienttemperature. Subsequently, switches 10, 10 a, and 14 would be configuredin the measurement position (in contrast to the switch position requiredby SCM steps 211 and 215), throughout the remaining session ofoperation, including operation in the operational mode, with thermistors3 and 4 also configured for measurements in said operation in theoperational mode.

Additionally, referring to FIG. 1, note that despite the above describeduse during SCM and the operational mode of internal switches 10, 10 a,and 14, operation of in RCM still requires that computer means 20automatically control current flow, associated with said internalswitches, during RCM measurements, taking place over a range of multipleambient temperatures. This is addressed in FIG. 9, depicting externalswitches for use with the preferred embodiment of FIG. 1, duringoperation in RCM, facilitating the use of entirely mechanical internalswitches 10, 10 a, and 14, and eliminating the need during RCM forcomputer means 20 to automatically operate internal switches 10, 10 a,and 14 in successive cycles of operation in RCM, at different ambienttemperatures. Connection points 101, 102, 103, 104, 105, 106, and databus 107, for connection with internal computer means 20, are provided asshown, so that RCM, performed at the factory, would use these saidconnection points to permit internal computer means 20 to automaticallyoperate external electronic switches 10′, 10 a′, and 14′, in place ofinternal switches 10, 10 a, and 14, respectively (while internalswitches 10, 10 a, and 14 are configured to be in an “open” state, i.e.,with no poles of said internal switches connected to inputs of anyamplifier 7, 7 a, or 13). Furthermore, if internal switches 10, 10 a,and 14 are mechanical switches, it is conceivable that external switches10′, 10 a′, and 14′ may be of electromechanical design, in order to moreclosely approximate the electrical behavior of mechanical switches, andeliminate other inaccuracies associated with solid state electronicswitches, while retaining the ability to be automatically controlled bycomputer means 20. That is, said external switches would be used inplace of internal mechanical switches 10, 10 a, and 14 during operationin RCM, so that said internal switches 10, 10 a, and 14 could be ofmechanical design, providing lower cost and simplicity.

Operational Mode (OM)

The embodiment of FIG. 1 is operated in the operational mode (OM) afterSCM has been completed. In OM, the translated, time drift correctedcurves from the most recent operation in SCM, along with a measuredambient temperature, are used to facilitate instantaneous thermalcompensation, on every temperature difference measurement. Recall thatoperation in SCM compensates for time drift of RCM generated thermaloffset curves, preferably at the outset of a session of operation,taking measurements at a single ambient temperature, and subsequentlypermitting substantially continuous operation in OM, utilizing said timedrift corrected RCM generated offset curves. OM involves four steps,which are continuously repeated, as described below. Additionally,throughout OM, switch 10 is configured to connect bridge nodes 11 and 12to difference temperature amplifier inputs 8 and 9, respectively, of thefirst difference temperature amplifier 7. Additionally, during OM,switch 10 a is configured to connect bridge nodes 12 and 11 todifference temperature amplifier inputs 8 a and 9 a, respectively, ofthe second difference temperature amplifier 7 a. Additionally, switch 14is configured to connect bridge node 11 to ambient temperature amplifier13. Switches 10, 10 a, and 14 are held in this configuration for theduration of OM, which in the preferred embodiment of FIG. 1 is theremainder of the current session of operation (while power is applied tothe system components of the preferred embodiment). A flow diagram,generally representing each of the individual steps involved inoperation of the preferred embodiment of the present invention in theoperational mode, is shown in FIG. 12C.

OM step 1 (220 in FIG. 12C): Referring to FIG. 1, in this OM step 220,computer means 20 determines the current ambient temperature, asmeasured by ambient temperature amplifier 13.

FIG. 10 depicts the same section of the graphs of FIGS. 5 and 8.However, note that RCM generated ambient temperature curve 25 does notappear (nor does RCM generated ambient reference curve 26). Instead, thecoordinate system of FIGS. 5 and 8 has been shifted horizontally, byamount final HT 45 (determined during SCM step 213), such that SCMtranslated ambient temperature curve 41 b is used to correlate ambienttemperature to RCM difference temperature curve 27 and to RCM differencereference curve 28 (SCM translated ambient reference curve 42 b is notused during OM, but is depicted in FIG. 10 for reference). Also, notethat the shift in coordinate system, by the amount final HT 45, causesendpoints on RCM generated curves 27 and 28 to be shifted, such that tothe left, of the left end of the ambient temperature range (i.e., to theleft of line 77), RCM generated curves 27 and 28 do not possess storedvalues. Therefore, the choice of ambient temperature range over whichRCM generated curves are recorded preferably takes such potential shifts(as that depicted in FIG. 10 to the left of line 77) into consideration,so that operation of the preferred embodiment of FIG. 1 is supportedover a desired range of temperatures, which may be subject to saidpotential shifts.

In order to determine the current ambient temperature, an ambienttemperature measurement is made, as previously described, according tothe preferred embodiment of FIG. 1, using ambient temperature amplifier13. Referring to FIG. 10, depicting the same section of the graph ofFIGS. 5 and 8, shifted as described above, a current ambient temperaturemeasurement is identified as point 61 and is hereafter referred to asmeasured ambient temperature point 61, situated on SCM translatedambient temperature curve 41 b. Additionally, an interpolation fractionis associated with measured ambient temperature point 61 (the use ofinterpolation fractions is described above in SCM step 212), based uponthe closest calibration points to said ambient temperature point 61 onSCM translated ambient temperature curve 41 b (points 61 a and 61 b).The interpolation fraction associated with ambient temperature point 61(40% in FIG. 10) is stored in memory 19.

OM step 2 (221 in FIG. 12C): In this OM step 221, computer means 20(FIG. 1) determines the actual temperature difference measurementbetween thermistors 3 and 4 (FIG. 1), at the current measured ambienttemperature, determined above in OM step 220. Thermistors 3 and 4 aredeployed in different thermal environments or at different thermalcontact positions to perform temperature difference measurements, oralternatively thermistors 3 and 4 are in thermal contact to producerandom thermal difference measurements between said thermistors 3 and 4.

First, computer means 20 preferably finds the larger of points on RCMdifference temperature curves 27 and 27 a (FIG. 2), which are associatedwith the above measured ambient temperature point 61 on SCM translatedambient temperature curve 41 b (FIG. 10), acquired during OM step 220.In FIG. 10, the above said associated points on the RCM differencetemperature curve 27 (curve 27 is assumed to possess the larger oftemperature difference points, compared to curve 27 a in the presentexample), associated with said points 61 a and 61 b, are identified aspoints 62 a and 62 b, respectively. Next, computer means 20 uses theinterpolation fraction, determined during OM step 220, above, (40%) todetermine the expected point on RCM temperature difference curve 27,identified as point 62 in FIG. 10, which represents the expecteddifference voltage for the current ambient temperature, if thermistors 3and 4 were at the same temperature. Use of said interpolation fractionis as described above in SCM step 212. Said expected point is hereafterreferred to as expected zero difference temperature (EZDT) 62.

Next, referring to FIG. 1, a measurement of the difference betweenbridge nodes 11 and 12 is performed by difference temperature amplifiers7 and 7 a. The largest of these is stored in memory 19, along with areference to the amplifier (7 or 7 a) which acquired it. Referring toFIG. 10, the stored measurement of the difference between bridge nodes11 and 12 is hereafter referred to as the measured differencetemperature (MDT) and is identified as MDT 63. For the purpose of thepresent description, MDT 63 is placed as shown in line with points 61and 62.

Next, computer means 20 adds the value last stored for DO 51 (determinedduring the last operation during SCM step 215) to the value (referringto FIG. 10) associated with EZDT 62, and the result is the compensatedexpected zero difference temperature (CEZDT), identified as point 64 inFIG. 10. This is the expected difference temperature measurement for thecurrent ambient temperature, if both thermistors 3 and 4 were at thesame temperature, compensated for difference measurement component drift(DO 51). It is noted that in the preferred embodiment, DO 51 is appliedto any measured EZDT 62 in this OM step 221, in order to arrive at acorresponding value for CEZDT 64, and therefore, due to thesubstantially linear drift of offset curves over time, DO 51, asdetermined in SCM step 215 at an arbitrary ambient temperature, can beregarded as a vertical translation of the difference temperature curveover the full range of ambient temperatures over which differencetemperature curve 27 was acquired during operation in RCM, as well as avertical translation of the difference reference curve 28, whichresulted in a translated difference reference curve, as was describedabove in connection with SCM step 215. It will be appreciated by thoseskilled in the art that if, as described above for the preferredembodiment, DO 51 is a single value throughout the range of ambienttemperatures over which RCM curves were acquired during operation inRCM, then the additional software and memory overhead required tocalculate and store a translated difference reference curve from whichto determine values for DO at different ambient temperatures within saidrange of ambient temperatures is not required, and instead only the saidsingle value of DO 51 needs to be stored in memory, since saidtranslated difference reference curve is determined only by adding saidsingle value of DO 51 to the difference reference curve 28, as wasdescribed above in connection with SCM step 215. Referring to FIG. 10,the discrepancy between the value associated with CEZDT point 64 and MDT63 represents a difference temperature between thermistors 3 and 4,compensated for component drift, and is hereafter referred to asadjusted difference temperature (ADT) and is identified as 72 in FIG.10. Cumulative error values associated with the difference temperaturemeasurement are discussed below in OM step 222.

OM step 3 (222 in FIG. 12C): In this OM step 222, error valuesassociated with the above measurements are consolidated, to yield acumulative error associated with the adjusted difference temperature(ADT) 72, determined above during OM step 221.

Referring to FIG. 11, recall that associated with the RCM generatedcurves, is an error associated with the time drift of components,including feedback resistors, bridge resistors, and thermistors, whichhas the effect of changing the “shape” of RCM generated curves, overtime. This error is referred to as TE, and was described in detail aspart of SCM step 213. Recall, also, that TE as it affects the ambienttemperature measurement, determined as described in SCM step 213, wasassociated with translated ambient temperature curve 42 b, and it wasdemonstrated, in connection with FIG. 6, that said TE associated withthe ambient temperature measurement, and initially associated withtranslated ambient temperature curve 42 b in SCM step 213, could also beapplied to SCM translated ambient temperature curve 41 b. A hypotheticalTE, associated with the ambient temperature measurement in the presentexample, is identified by ambient temperature TE error bar (ATE) 65 inFIG. 11.

Additionally, recall that in the discussion of TE, in SCM step 213,there were also mentioned similarly derivable additional TE's, for RCMgenerated difference temperature curve 27 and difference reference curve28. These additional TE's can be quantified as described generally inconnection with FIG. 7 (as part of SCM step 213 for a hypothetical curve55). Referring to FIG. 11, these additional TE's associated withrelating points on SCM translated ambient temperature curve 41 b toother RCM generated curves include: a differential temperature TE,associated with the process of relating a point on SCM translatedambient temperature curve 41 b with a corresponding point on differencetemperature curve 27, hereafter referred to as difference temperatureTE, identified in FIG. 11 as TE error bar 67; and a difference referenceTE, associated with the process of relating a given point on SCMtranslated ambient temperature curve 41 b with a corresponding point ondifference reference curve 28, hereafter referred to as differencereference TE, identified in FIG. 11 as TE error bar 66. Referring toFIG. 11, note that TE error bars 66 and 67 represent hypothetical valuesto illustrate the method of the present invention. Finally, there is thehypothetical error value associated with DTDR 53 (shown in FIG. 8 as twoLSB's of A/D converter 17, and not shown in FIG. 11), described above inSCM step 215. In OM step 222, the above error terms are allconsolidated, to yield a final difference error (FDE) directly relatedto the difference temperature measurement, as described below.

Referring to FIG. 11, in order to be applicable to a final differencetemperature measurement, the final consolidation of error termspreferably comprises error terms which are all associated directly witha difference measurement related to either difference temperature curve27 or difference reference curve 28 (recall that in the present example,referring to FIG. 2, difference measurements are associated with thefirst difference temperature curve 27 and the first difference referencecurve 28, as opposed to the second difference temperature curve 27 a andthe second difference reference curve 28 a). All such error terms areeventually consolidated into an error bar that preferably surrounds themeasured difference temperature point (MDT) 63. DTDR 53 and TE's 66 and67 are already directly associated with either difference temperaturecurve 27, or difference reference curve 28, and therefore are alreadydirectly applicable to a final temperature difference measurement.However, error constituents, associated with ambient temperaturemeasurements (i.e., ATE 65) must be converted into its direct impactupon the temperature difference measurements. Recall that over any givenambient temperature range, difference temperature measurements of thepreferred embodiment of FIG. 1 typically vary two to three orders ofmagnitude more slowly than ambient temperature measurements, over theoperating ambient temperature range of the preferred embodiment.Therefore, the error value associated with ATE 65, which represents anerror in the ambient temperature measurement, will typically beattenuated by two to three orders of magnitude, when quantified for itseffect upon difference temperature measurements. Additionally, note, asmentioned above, that the slow variation of thermal offset curves,relative to ambient temperature, as depicted in the figures isnecessarily exaggerated, in order to illustrate the method of thepresent invention.

Once converted into its direct effect on difference temperaturemeasurements, the effect of TE error bar ATE 65 on temperaturedifference measurements is added to DTDR 53 and TE's 66 and 67 bycomputer means 20, in order to provide a final cumulative differenceerror value, or final difference error (FDE) 68 in FIG. 11, surroundingMDT 63, as shown. FDE 68, for any given difference temperaturemeasurement, represents the uncertainty, or potential error, in saidtemperature difference measurement.

As described above, referring to FIG. 11, the error component, relatedto ambient temperature measurement, is represented by ATE 65. Referringto FIG. 11, the procedure in OM step 222 to calculate the contributionof ATE 65 to the final difference error measurement (FDE) 68 is asfollows. The computed value for ATE 65 is preferably first applied toSCM translated ambient temperature curve 41 b, such that the currentambient temperature measurement point 61 in the present example issituated at the midpoint of ATE 65, as shown. The resulting ATEendpoints include a first ATE endpoint 70 and a second ATE endpoint 71.Said ATE endpoints are next each associated with the nearest recordedcalibration points on SCM translated ambient temperature curve 41 b,along with an interpolation fraction, as described above in SCM step 212for each of said first and second ATE endpoints 70 and 71. Consequently,there will be a calculated first interpolation fraction, along with twofirst associated points 70 a and 70 b on SCM translated ambienttemperature curve 41 b, for first ATE endpoint 70; and a calculatedsecond interpolation fraction, along with two second associated points71 a and 71 b, as illustrated in FIG. 11, on SCM translated ambienttemperature curve 41 b, for second ATE endpoint 71, as shown in FIG. 11.The resulting two said first associated points 70 a and 70 b on SCMtranslated ambient temperature curve 41 b are then used to find theirassociated recorded calibration points 73 a and 73 b on RCM differencetemperature curve 27. The same process is repeated to determine recordedcalibration points 74 a and 74 b on RCM difference temperature curve 27,associated with the two said second associated points 71 a and 71 b onSCM translated ambient temperature curve 41 b.

Next, referring to FIG. 11, the first and second interpolationfractions, associated above with each of first and second ATE endpoints70 and 71, respectively, are now used to find the exact position of saidpoints 73 and 74 on difference temperature measurement curve 27 that areassociated with said ATE endpoints 70 and 71, respectively, using saidinterpolation fractions by the method described in SCM step 212. Thevalue of the arithmetic difference between the vertical axis values ofpoints 73 and 74 on difference curve 27 is the magnitude of ATE that isapplied to FDE 68, hereafter referred to as difference ATE (DATE),identified in FIG. 11 as DATE 69. The magnitude of DATE 69 is then addedto DTDR 53 (determined during SCM step 216 and depicted in FIG. 8), aswell as to TE 66 and TE 67, to equal the final value of FDE 68, as shownin FIG. 11.

OM step 4 (223 in FIG. 12C): Referring to FIG. 11, the differencebetween the value associated with CEZDT 64 (determined during OM step221) and the measured difference value, represented by MDT 63(determined during OM step 221) in FIG. 11, represents a differencetemperature between thermistors 3 and 4 (ADT 72 in FIG. 11), compensatedfor component drift. This value is considered to be accurate to withinthe value of FDE 68, determined above, during OM step 222. FDE 68 ispreferably used to control the way that difference temperature ADT 72 isreported to the user by computer means 20. For example, if aninstantaneous FDE value is determined to be equal to four LSB's of A/Dconverter 17 (FIG. 1), then reported temperature differences should bein increments of not less than four LSB's of A/D converter 17, and thisresolution limit is preferably reported to the user, for example, ondigital display 20 a, along with the compensated temperature differencemeasurement ADT 72.

It will be appreciated by those skilled in the art that differencemeasurements determined by the method of the present invention, andquantified in terms of LSB's of an analog to digital converter (e.g.,A/D converter 17, FIG. 1), can be correlated to actual values of aphysical quantity, such as temperature. In the case of temperature, forexample, referring to FIG. 1, this correlation may be accomplished bycalculating the effect of a hypothetical temperature difference betweenthermistors 3 and 4, taking bridge voltage at point 22 into account, andnoting the resulting difference voltage that would appear at the outputsof amplifiers 7 and 7 a, as a result of said hypothetical temperaturedifference. Additionally, it is conceivable that such a correlationbetween LSB's of an analog to digital converter, and actual values ofdifferences in a physical quantity, such as temperature, can be madeempirically, by intentionally applying a known difference in saidphysical quantity to the difference measurement system of the presentinvention. For example, in the case of temperature differencemeasurements, referring again to FIG. 1, if a known temperaturedifference of 0.1° C. is intentionally applied between thermistors 3 and4, at a given ambient temperature, and said known temperature differenceof 0.1° C., results in a difference measurement, determined by themethod of the present invention, of 1000 LSB's of A/D converter 17, thena numerical correlation factor between LSB's of A/D converter 17, atleast at the above mentioned given ambient temperature, and actualtemperature differences between thermistors 3 and 4 can be made, which,in the case of the present example, would be 1000 LSB's/0.1° C., or 100LSB's/micro-degree C. It will be appreciated by those skilled in the artthat additional intentionally applied differences in an above saidphysical quantity (e.g., in the above example, at 1° C., and 0.01° C.,in addition to 0.1° C.) can be applied to achieve a greater range ofmeasurement and/or greater accuracy, and that such correlations may bemade at the time of operation in RCM, at specific ambient conditions, orover a range of expected ambient conditions. The latter case ofintentionally applying at least one known difference in a physicalquantity over a range of ambient conditions would result in at least onecurve relating said known difference in a physical quantity tomeasurements in LSB's of an analog to digital converter (e.g., A/Dconverter 17 in FIG. 1) over said range of ambient conditions, such thatany given ambient condition within the said range of ambient conditions,on said curve, corresponds to a correlation factor relating LSB's of ananalog to digital converter, at the above given ambient condition, tosaid known difference in a physical quantity. Then, during operation inSCM, using the above said curve, it is conceivable that the said knowndifference in a physical quantity can be intentionally applied onceagain, at a given ambient condition determined at the time of operationin SCM, and the resulting difference measurement in LSB's of said analogto digital converter compared to the recorded difference measurement inLSB's, on the above said curve, recorded at the time of operation inRCM, while the said known difference in a physical quantity was beingintentionally applied, at the above said given ambient condition, inorder to update the correlation factor between differences in saidphysical quantity, and the resulting difference measurements in LSB's ofsaid analog to digital converter. Such an updated correlation factor,referred to below as C₁, determined during operation in SCM, at theabove said given ambient condition determined at the time of operationin SCM, can be applied to all subsequent measurements during operationin OM, or the said updated correlation factor C₁ can be related to thecorrelation factor referred to below as C₀, determined at the time ofoperation in RCM (and associated with the above said given ambientcondition, on the above said curve), to provide a correlation factorcorrection, referred to below as X_(c)=C₁/C₀. Subsequently, duringoperation in OM, the above said correlation factor correction X_(c), canbe applied to other correlation factors, on the above said curve, suchother correlation factor being referred to below as C₀′, associated withan ambient condition on said curve, said ambient condition determinedduring operation in OM. For example, if at a given ambient condition,determined during operation in OM, the expected correlation factor isC₀′ on the above said curve (said curve determined at the time ofoperation in RCM), the corrected correlation factor at the said givenambient condition would be C₁′=X_(c) * C₀′, with the correlation factorcorrection X_(c) determined as described above. It is also conceivablethat during OM, at least one known difference in a physical quantity canbe intentionally applied, at any time, as described above, to yieldcorrelation factors usable at ambient temperatures determined duringoperation in OM.

It will be appreciated that the embodiments of the present inventiondescribed above are susceptible to various modifications, changes, andadaptations. For example, other differential measurement systems intowhich differential signals are connected, that can readily be made tohave sufficiently equal measurable values, in order to facilitateoperation in RCM and SCM (as described above), such as amplifiers withdifferential inputs held at substantially equal values during operationin RCM and SCM by, for example, connecting the differential inputstogether, or connecting them to a reference signal, or by positioningsensors such that they are subject to substantially the same excitationof a physical variable to which said sensors are susceptible, saidinputs at other times, specifically during OM, being connected to otherelectronics having differential signals, for example, to fixedresistance bridge circuits, circuits having differential currents (e.g.,a differential electrometer), and circuits employing sensors other thantemperature sensors, can readily be adapted for use in association withthe drift compensation method of the present invention. In the latercases, in which differences in signals, not representative oftemperature (e.g., light intensity, pressure, or other physicalvariable), are being measured, and in which component drift due totemperature variation is not significant, time drift compensation canconceivably be accomplished by replacing measurement bridge 2 in FIG. 1with another type of sensing electronics, similarly having two outputs.For example, two photodiode detection circuits, of the type well knownto persons skilled in the art, for measuring differential lightintensity, may replace measurement bridge 2 in FIG. 1, with the twooutputs, from said two photodiode detection circuits, connected todifference amplifiers 7 and 7 a in FIG. 1, with one of said outputsconnected to ambient amplifier 13. That is, in this case of theembodiment for measuring differential light intensity, ambienttemperature amplifier 13 in FIG. 1 would measure ambient lightintensity, and difference temperature amplifiers 7 and 7 a, connected tosaid photodiode circuits, would instead measure differential lightintensity. In this case, operation of the present invention will be thesame as described earlier, except that rather than cycling such anembodiment for measuring differential light intensity through a range ofambient temperatures, during operation in RCM, said embodiment wouldinstead be cycled through a range of expected ambient light intensities,with both of the photodiode circuits configured to experience anidentical level of light intensity, during operation of said embodimentin both RCM and in SCM. That is, rather than compensating variations indifference temperature measurement, due to ambient temperaturevariations, variations in the light intensity difference measurements,due to variations in the level of ambient, or “common mode”, lightintensity would be compensated, along with component time drift.Additionally, in the case of the embodiment for measuring differentiallight intensity, the set of RCM generated curves associated with saidembodiment, rather than correlating ambient temperature to expected zerodifference temperature, would instead correlate ambient light intensityto expected zero difference light intensity. In the case of such analternative embodiment of the present invention, for measuringdifferential light intensity, and in which both time and temperaturedrift are considered significant, but in which variations in measureddifferential light intensity do not vary significantly with ambient(common mode) light intensity, measurement bridge 2 in FIG. 1 wouldstill be replaced by said photodiode circuits, for measuring differencesin light intensity, and difference temperature amplifiers 7 and 7 a inFIG. 1 would still measure differential light intensity. However,ambient temperature amplifier 13 in FIG. 1 would measure ambienttemperature, independently of the differential light intensitymeasurement, and independently of associated photodiode circuitry, and,moreover, it will be appreciated that the set of RCM generated curvesassociated with such an embodiment, rather than correlating ambienttemperature to expected zero difference temperature, as in theembodiment depicted in FIG. 1, would instead correlate ambienttemperature to expected zero difference light intensity. It will beappreciated by those skilled in the art that such an embodiment of thepresent invention could use any ambient temperature measurementtechnique, keeping in mind, however, that the effect of time andtemperature drift of said ambient temperature measurement technique,upon differential measurements, are typically attenuated by two to threeorders of magnitude, since variations in differential measurements, dueto ambient temperature variations, are typically two to three orders ofmagnitude smaller than said ambient temperature variations, as describedearlier. Finally, it is conceivable that the above alternativeembodiments compensate for both temperature drift and ambient (commonmode) light intensity, as well as time drift. In this case, referring toFIG. 13, ambient light amplifier 301 would measure ambient lightintensity, via at least one of photodiode circuits 302 or 303 (302 isused in the preferred embodiment of FIG. 13), and a separate ambienttemperature amplifier 304 would be employed to measure ambienttemperature, via a separate thermistor bridge comprising thermistor 3and resistance 23, preferably powered by bridge voltage 21, with saidambient temperature amplifier 304 connected to A/D converter 17. Also,note that the inverting input of ambient temperature amplifier 304 isconnected to node 16 of substantially time stable resistance bridge 1,and the non-inverting input of said ambient temperature amplifier 304 isconnected to either thermistor bridge node 11 (between thermistor 3 andresistance 23), or node 15 of substantially time stable resistancebridge 1, depending on the state of switch 305. It will be appreciatedby those skilled in the art that while switch 305 may be operated toconnect the non-inverting input of ambient temperature amplifier 304 tothermistor bridge node 11, for the purpose of measuring ambienttemperature, said switch 305 may also be operated to connect node 15 ofsubstantially time stable resistance bridge 1 to the non-inverting inputof said ambient temperature amplifier 304 in order to calibrate saidambient temperature amplifier 304 for time drift in accordance with themethod of the present invention. During operation of such an embodimentin RCM, with both photodiode circuits exposed to identical lightintensity, a distinct set of characteristic RCM curves would begenerated over a range of ambient temperatures, each said set of RCMcurves corresponding to one of at least two ambient light intensities,preferably including the minimum and maximum expected ambient lightintensities, as detected by ambient light amplifier 301. This wouldresult in at least two sets of RCM generated curves, each said set ofcurves corresponding to a particular ambient light intensity, and eachsaid set of curves correlating ambient temperature to expected zerodifference light intensity. In this case, during operation in SCM, anambient light intensity measurement would be used to select which set orsets of RCM generated curves is to be used (in subsequent operation inOM) to correlate ambient temperature to differential light intensity.Additionally, said selected set or sets of RCM generated curvescorresponding to the measured ambient light intensity (determined duringoperation in SCM) would also be that set or sets of RCM generated curvesthat are compensated for component time drift, as described above in thediscussion of SCM. It will be appreciated that if the measured ambientlight intensity is between values of ambient light intensity, for whichsets of RCM generated curves were generated during operation in RCM,then the two sets of RCM generated curves associated with values ofambient light intensity, closest to that ambient light intensityactually measured during operation in SCM, can be used to interpolate aset of RCM curves, approximating that which should be associated withsaid measured ambient light intensity. Approaches that mathematicallymodel multiple curves, that can be applied to such a multiple curveinterpolation problem, are well known, for example, see“Piecewise-linear interpolation between polygonal slices” by GillBarequet and Micha Sharir in Proceedings of the Tenth Annual Symposiumon Computational Geometry, pages 93-102. Additionally, referring againto FIG. 1, it will be appreciated by those skilled in the art that themethod for measuring differential temperature there depicted preferablypositions thermistors 3 and 4 of measurement bridge 2 in thermal contactwith amplification electronics, including at least the most thermallysensitive of amplifiers 7, 7 a, and 13, so that ambient temperaturemeasurements reflect an ambient temperature common to both measurementbridge 2 and said amplification electronics. It is conceivable thatmeasurement bridge 2 and said amplification electronics are locatedsufficiently far from each other that distinct ambient temperaturecurves are preferably acquired and recorded for the measurement bridge 2(FIG. 1) and for the amplification electronics, including at least themost thermally sensitive of amplifiers 7, 7 a, and 13. For the case ofdifferential thermal measurement, referring again to FIG. 13, photodiodecircuits 302 and 303 would be replaced by a measurement bridgecomprising thermistors such as those depicted in measurement bridge 2shown in FIG. 1, and ambient light amplifier 301 (FIG. 13) would insteadmeasure ambient temperature at said measurement bridge (just as isperformed by ambient temperature amplifier 13 shown in FIG. 1), andseparate ambient temperature amplifier 304 would be employed to measureambient temperature via a separate thermistor bridge preferablycomprising thermistor 3 and resistance 23 (FIG. 13), with saidthermistor 3 (FIG. 13) preferably positioned in substantial thermalcontact with thermally sensitive amplification electronics, including atleast the most thermally sensitive of amplifiers 7, 7 a, 301, and 304.Again, it will be appreciated by those skilled in the art that if themeasurement bridge ambient temperature is between values of themeasurement bridge ambient temperature for which sets of RCM generatedcurves were generated during operation in RCM, then the two sets of RCMgenerated curves associated with values of measurement bridge ambienttemperature closest to that measurement bridge ambient temperatureactually measured during operation in SCM can be used to interpolate (bya known approach such as disclosed in the interpolation reference citedabove) a set of RCM curves, approximating that which should beassociated with said measurement bridge ambient temperature actuallymeasured during operation in SCM. Said interpolated set of RCM curveswould then correlate ambient temperature, as measured by ambienttemperature amplifier 304, to measurements associated with each curve inthe said set of RCM curves. By extension, it is conceivable, and will beappreciated by those skilled in the art, that additional sets of RCMgenerated curves corresponding to ambient conditions of physicalvariables at additional locations in the circuit can be acquired andcombined by known means (such as disclosed in the interpolationreference cited above) to arrive at a single composite curve for eachcurve common to said sets of RCM generated curves. Additionally, it willbe apparent to those skilled in the art that, generally speaking,difference measurements performed by the method of the present inventionare referenced to an ambient condition, so that time drift ofelectronics associated with ambient condition measurements will have aneffect upon the accuracy of difference measurements. Nevertheless, ithas been pointed out that a given difference measurement performed bythe method of the present invention varies more slowly, over a givenrange of ambient conditions, than ambient condition measurements overthe same given range of ambient conditions. Consequently, ambientcondition measurements need not be performed with as much accuracy asdifference measurements, in order to achieve a given differencemeasurement accuracy. Therefore, it will be appreciated by those skilledin the art that compensation for time drift of electronic componentsassociated with ambient condition measurements is not as critical toaccurate difference measurements as compensation for time drift ofelectronic components associated with difference measurements versusmeasured ambient condition. Accordingly, it is conceivable that in somecases the embodiments of the present invention may be simplified suchthat difference measurements associated with the method of the presentinvention correlate directly to ambient condition measurements, and thatthe method of translating difference offset curves described above maybe conducted without the need to acquire and translate an ambientreference curve, or to reference said difference measurements to atranslated curve representative of ambient condition compensated fortime drift of electronic components associated with ambient conditionmeasurements versus measured ambient condition, with the result thatsaid difference measurements can instead be referenced directly tomeasured ambient conditions, uncompensated for time drift, as measureddirectly by electronics associated with ambient condition measurements.In this case, difference measurements are compensated for time drift ofcomponents associated with difference measurement, again utilizing thesubstantially linear drift of offset curves over time, in order topermit measurements at an arbitrary ambient condition during operationin SCM, in accordance with the preferred embodiment, to substantiallycompensate subsequent difference measurements during operation in OM,over the range of ambient conditions in which said offset curves wereacquired during operation in RCM. More specifically, referring to thepreferred embodiment of FIG. 1, in the case in which only differencemeasurements are compensated for drift, rather than both difference andambient temperature measurements, RCM step 203, used for acquiring RCMgenerated ambient reference curve 26 (FIG. 2), can be eliminated, sincethis curve is used only to compensate ambient temperature measurements,and is consequently not needed. Also, referring to FIG. 8, allsubsequent operations that reference the translated ambient temperaturecurve 41 b instead reference the uncompensated ambient temperature curve25 directly. Finally, in the case in which only difference measurementsare compensated for drift, rather than both difference and ambienttemperature measurements, SCM steps 2, 3, and 4 (211, 212, and 213,respectively, in FIG. 12B) are skipped, because these steps are usedonly to compensate ambient temperature measurements for drift ofcomponents associated with ambient temperature measurement. Otherwise,in the case in which only difference measurements are compensated fordrift, rather than both difference and ambient temperature measurements,the preferred compensation technique of the present invention is aspreviously described. Additionally, other high resolution differentialmeasurement systems designed specifically to measure near-equaldifferential signals, known to average to zero over time, for thepurpose of generating natural random numbers (in contrast topseudo-random numbers), such as signals resulting from diode noise, canalso benefit from, and may be readily adapted to, the method of thepresent invention. Accordingly, the scope of the present invention canonly be ascertained by reference to the appended claims.

What is claimed is:
 1. A method for compensating electronic differencemeasurement apparatus for at least one of time and temperature drift ofelectronic components, comprising the steps of: providing a first signalrepresentative of a first value of a physical variable; providing asecond signal representative of a second value of the same physicalvariable; providing at least one difference signal amplification meansfor amplifying the difference between the first and second signals toproduce a difference signal; providing at least one ambient conditionsignal means responsive to the ambient condition of at least one of a)the physical variable and b) temperature for providing a third signal;providing analog to digital converter means for converting thedifference signal produced by the difference signal amplification meansand the third signal provided by the ambient condition signal means intodigital form; providing computer means for compensating the electronicdifference measurement apparatus for drift of electronic components;providing memory means for storing calibration information used forcompensating the electronic difference measurement apparatus for driftof electronic components; operating in a reference calibration mode, inwhich at least one offset curve representative of offset for electroniccomponents associated with difference signal measurements versusmeasured ambient condition is acquired over a range of ambientconditions and stored in the memory means, the curve so generated beingreferred to as a difference reference curve; operating in a standardcalibration mode, in which a difference signal offset that substantiallycompensates the difference reference curve for drift over time isdetermined at a current arbitrary ambient condition by comparingdifference measurements at the current arbitrary ambient condition topreviously stored values on the difference reference curve and at leastone of a) translating the difference reference curve in order tosubstantially compensate the difference reference curve for time driftassociated with difference signal measurement, the difference referencecurve after translation being referred to as a translated differencereference curve, the translated difference reference curve being used toprovide a difference signal offset during an operational mode, thatsubstantially compensates difference measurements for drift, and b)setting the difference signal offset to the difference between anestimated difference reference and an actual difference reference, thedifference signal offset being used to linearly translate differencemeasurements during the operational mode in order to substantiallycompensate difference measurements for drift of components associatedwith difference measurement; and performing at least one differencemeasurement in the operational mode, in which a measurementrepresentative of current ambient condition provides an ambientcondition measurement which is correlated to a difference signal offsetmeasurement, the difference signal offset measurement being at least oneof a) determined from the translated difference reference curve at themeasured ambient condition measurement, b) equivalent to the differencesignal offset determined during the standard calibration mode, and c)derived by empirically comparing values for the difference signal offsetat various times during operation in the standard calibration mode atvarious ambient conditions, and the difference signal offset measurementbeing used to correct the difference signal for component drift toprovide a compensated difference measurement between the first andsecond values of the physical variable.
 2. A method as defined in claim1 wherein the difference signal amplification means comprisesdifferential inputs connected to a measurement bridge comprising a firstsensor having an impedance responsive to the physical variable, thefirst sensor being connected in series to a first impedance, the firstsensor and first impedance being connected across a measurement bridgepotential for producing the first signal, and the first sensor and firstimpedance being connected in parallel to a second sensor having animpedance responsive to the same physical variable as the first sensor,the second sensor being connected in series with a second impedance, theseries-connected second sensor and second impedance also being connectedacross the measurement bridge potential for producing the second signal,and the inputs to the difference signal amplification means beingconnected to the measurement bridge, such that each differential inputis connected to the measurement bridge at a different bridge node,situated between either the first sensor and first impedance or betweenthe second sensor and second impedance.
 3. A method as defined in claim2 wherein the first and second sensors are configured to be subject tosubstantially the same value of the physical variable, to which thesensors are responsive, during the reference calibration mode, as wellas during the standard calibration mode, and wherein operation of theelectronic difference measurement apparatus in the reference calibrationmode, with the first and second sensors subject to substantially thesame value of the physical variable, results in at least one physicalvariable difference curve distinct from the difference reference curve,which is used during the standard calibration mode, with the first andsecond sensors again subject to substantially the same value of thephysical variable, to compare points on the physical variable differencecurve, acquired at different times, so as to provide an offsetmeasurement which substantially compensates for variations between thesensors, over the range of ambient conditions in which the physicalvariable difference curve is acquired, and additionally to compensatefor time drift in the variation over the ambient condition range inwhich the physical variable difference curve was acquired during thereference calibration mode.
 4. A method as defined in claim 3 whereinthe measurement bridge potential is applied across a referenceresistance bridge to generate at least one reference signal used tocompensate the ambient condition signal means for component time drift.5. A method as defined in claim 4 wherein the potential across thereference resistance bridge is at least one of a) applied to an input ofthe analog to digital converter means in order to provide a ratiometriccompensation for variations in bridge voltage and b) generated using theanalog to digital converter means.
 6. A method as defined in claim 3wherein at least one input to the ambient condition signal means isconnected to one of the first and second sensors of the measurementbridge.
 7. A method as defined in claim 3 wherein the first and secondsensors and the ambient condition signal means are responsive totemperature.
 8. A method as defined in claim 7 wherein at least oneinput to the ambient condition signal means is connected to one of thefirst and second temperature sensors of the measurement bridge.
 9. Amethod as defined in claim 3 wherein at least one of the differencereference curve and physical variable difference curve is re-acquired ata known time, relative to the time at which the respective offset curveswere last acquired, and compared to previous versions of the respectiveoffset curves to at least one of a) estimate error in the lineartranslations, associated with the standard calibration mode, and b)track trends in drift of the respective offset curves.
 10. A method asdefined in claim 9 wherein errors associated with the lineartranslations in the standard calibration mode are at least one of a)reported to a user and b) used to limit the accuracy with whichdifference measurements are reported to the user.
 11. A method asdefined in claim 3 wherein drift parameters associated with at least oneof the first sensor, second sensor, and difference signal amplificationmeans are stored in the memory means and used to determine at least oneof the a) difference reference curve and b) physical variable differencecurve.
 12. A method as defined in claim 2 wherein the measurement bridgepotential is applied across a reference resistance bridge to generate atleast one reference signal used to compensate the ambient conditionsignal means for component time drift.
 13. A method as defined in claim12 wherein the potential across the reference resistance bridge is atleast one of a) applied to an input of the analog to digital convertermeans in order to provide a ratiometric compensation for variations inbridge voltage and b) generated using the analog to digital convertermeans.
 14. A method as defined in claim 2 wherein at least one input tothe ambient condition signal means is connected to one of the first andsecond sensors of the measurement bridge.
 15. A method as defined inclaim 2 wherein the first and second sensors and the ambient conditionsignal means are responsive to temperature.
 16. A method as defined inclaim 15 wherein at least one input to the ambient condition signalmeans is connected to one of the first and second temperature sensors ofthe measurement bridge.
 17. A method as defined in claim 2 wherein thepotential across the measurement bridge is at least one of a) applied toan input of the analog to digital converter means in order to provide aratiometric compensation for variations in bridge voltage and b)generated using the analog to digital converter means.
 18. A method asdefined in claim 1 wherein the difference reference curve is re-acquiredand compared to a previous version of the difference reference curve toat least one of a) estimate error in the linear translations, associatedwith the standard calibration mode, and b) track trends in drift of thedifference reference curve.
 19. A method as defined in claim 18 whereinerrors associated with the linear translations in the standardcalibration mode are at least one of a) reported to a user and b) usedto limit the accuracy with which difference measurements are reported tothe user.
 20. A method as defined in claim 1 wherein at least one of thefirst and second signals provides a substantially random signal source.21. A method as defined in claim 1, further comprising the steps ofproviding a first sensor responsive to the first value of the physicalvariable for producing the first signal and providing a second sensorresponsive to the second value of the physical variable for producingthe second signal, wherein the first and second sensors are configuredto be subject to substantially the same value of the physical variable,to which the sensors are responsive, during the reference calibrationmode, as well as during the standard calibration mode, and whereinoperation of the electronic difference measurement apparatus in thereference calibration mode, with the first and second sensors subject tosubstantially the same value of the physical variable, results in atleast one physical variable difference curve distinct from thedifference reference curve, which is used during the standardcalibration mode, with the first and second sensors again subject tosubstantially the same value of the physical variable, to compare pointson the physical variable difference curve, acquired at different times,so as to provide an offset measurement which substantially compensatesfor variations between the sensors, over the range of ambient conditionsin which the physical variable difference curve is acquired, andadditionally to compensate for time drift in the variation over theambient condition range in which the physical variable difference curvewas acquired during the reference calibration mode.
 22. A method asdefined in claim 21 wherein drift parameters associated with at leastone of the first sensor, second sensor, and difference signalamplification means are stored in the memory means and used to determineat least one of the a) difference reference curve and b) physicalvariable difference curve.
 23. A method as defined in claim 1 whereinthe ambient condition signal means is responsive to temperature forproviding the third signal.
 24. A method as defined in claim 1 wherein afirst ambient condition signal means is responsive to the ambientcondition of a first physical variable to provide the third signal andat least one second ambient condition signal means is responsive to asecond physical variable for providing a fourth signal, the fourthsignal being converted by the analog to digital converter means intodigital form and stored in the memory means for use by the computermeans for compensating the electronic difference measurement apparatusfor drift of electronic components due to variations in the ambientvalue of the second physical variable, and wherein at least two sets ofoffset curves are acquired over the range of the first physicalvariable, each set of offset curves being acquired at a different valueof ambient condition of the second physical variable within the range ofambient conditions of the second physical variable.
 25. A method asdefined in claim 24 wherein at least one of first and second physicalvariables is temperature.
 26. A method as defined in claim 25 whereinmultiple second physical variables are representative of the samephysical parameter in different physical locations.
 27. A method asdefined in claim 24 wherein multiple second physical variables arerepresentative of the same physical parameter in different physicallocations.
 28. A method as defined in claim 24 wherein drift parametersassociated with at least one of the first sensor, second sensor, anddifference signal amplification means are stored in the memory means andused to determine at least one of the a) difference reference curve andb) offset curves.
 29. A method as defined in claim 1 wherein driftparameters associated with at least one of the difference signalamplification means and ambient condition signal means are stored in thememory means and used to determine the difference reference curve.
 30. Amethod as defined in claim 1, further comprising: operating in thereference calibration mode, in which offset curves are acquired over arange of ambient conditions and stored in the memory means, the offsetcurves comprising: a) at least one curve representative of offset forelectronic components associated with ambient condition measurementsversus measured ambient condition, the curve so generated being referredto as an ambient reference curve; and b) the difference reference curve;operating in the standard calibration mode, in which an ambient signaloffset and a difference signal offset that substantially compensate theambient reference curve and difference reference curve, respectively,for drift over time are determined at a current arbitrary ambientcondition by comparing measurements at the current arbitrary ambientcondition to previously stored values on the ambient reference curve anddifference reference curve and, for both the ambient reference curve anddifference reference curve, one of a) translating at least one of theambient reference curve and difference reference curve in order tosubstantially compensate the at least one reference curve for time driftassociated with ambient condition measurement and difference signalmeasurement, the respective reference curves after translation beingreferred to as a translated ambient reference curve and a translateddifference reference curve, respectively, and b) determining at leastone of an ambient signal offset and difference signal offset byobtaining the difference between an estimated ambient referencemeasurement and an actual ambient reference measurement, and between anestimated difference reference measurement and an actual differencereference measurement, respectively, the respective signal offset beingused to linearly translate ambient condition measurements and differencemeasurements, respectively, in order to substantially compensate therespective measurements during the operational mode for drift ofcomponents associated with ambient condition measurement and differencemeasurement, respectively; and performing at least one differencemeasurement in the operational mode in which a measurementrepresentative of current ambient condition compensated for time driftusing at least one of the translated ambient reference curve and ambientsignal offset determined during the standard calibration mode providesan ambient condition measurement which is correlated to a differencesignal offset measurement, the difference signal offset measurementbeing at least one of a) determined from the translated differencereference curve at the compensated ambient condition measurement, b)equivalent to the difference signal offset determined during thestandard calibration mode, and c) derived by empirically comparingvalues for the difference signal offset at various times duringoperation in the standard calibration mode, at various ambientconditions, and the difference signal offset measurement being used tocorrect the difference signal for component drift to provide acompensated difference measurement between the first and second valuesof the physical variable.
 31. A method as defined in claim 30 whereinthe ambient reference curve and difference reference curve arere-acquired and compared to previous versions of the respective offsetcurves to at least one of a) estimate error in the linear translations,associated with the standard calibration mode, and b) track trends indrift of the respective offset curves.
 32. A method as defined in claim30, further comprising the steps of providing a first sensor responsiveto the first value of the physical variable for producing the firstsignal, providing a second sensor responsive to the second value of thephysical variable for producing the second signal, storing driftparameters associated with at least one of the first sensor, secondsensor, difference signal amplification means, and ambient conditionsignal means in the memory means, and using the stored drift parametersto determine at least one of a) the ambient reference curve and b) thedifference reference curve.
 33. Apparatus for compensating electronicdifference measurements for at least one of time and temperature driftof electronic components, comprising: means for providing a first signalrepresentative of a first value of a physical variable; means forproviding a second signal representative of a second value of the samephysical variable; at least one difference signal amplification meansresponsive to the first and second signals for amplifying the differencebetween the first and second signals to produce a difference signal; atleast one ambient condition signal means responsive to the ambientcondition of at least one of a) the physical variable and b) temperaturefor providing a third signal; analog to digital converter meansconnected to the difference signal amplification means and the ambientcondition signal means for converting the difference signal produced bythe difference signal amplification means and the third signal providedby the ambient condition signal means into digital form; computer meansfor compensating the electronic difference measurement apparatus fordrift of electronic components; memory means for storing calibrationinformation used for compensating the electronic difference measurementapparatus for drift of electronic components; the apparatus beingoperable in a reference calibration mode, in which at least one offsetcurve representative of offset for electronic components associated withdifference signal measurements versus measured ambient condition isacquired over a range of ambient conditions and stored in the memorymeans, the curve so generated being referred to as a differencereference curve; the apparatus additionally being operable in a standardcalibration mode, in which a difference signal offset that substantiallycompensates the difference reference curve for drift over time isdetermined at a current arbitrary ambient condition by comparingdifference measurements at the current arbitrary ambient condition topreviously stored values on the difference reference curve and at leastone of a) translating the difference reference curve in order tosubstantially compensate the difference reference curve for time driftassociated with difference signal measurement, the difference referencecurve after translation being referred to as a translated differencereference curve, the translated difference reference curve being used toprovide a difference signal offset during an operational mode, thatsubstantially compensates difference measurements for drift, and b)setting the difference signal offset to the difference between anestimated difference reference and an actual difference reference, thedifference signal offset being used to linearly translate differencemeasurements during the operational mode in order to substantiallycompensate difference measurements for drift of components associatedwith difference measurement; and the apparatus for performing at leastone difference measurement in the operational mode, in which ameasurement representative of current ambient condition provides anambient condition measurement which is correlated to a difference signaloffset measurement, the difference signal offset measurement being atleast one of a) determined from the translated difference referencecurve at the measured ambient condition measurement, b) equivalent tothe difference signal offset determined during the standard calibrationmode, and c) derived by empirically comparing values for the differencesignal offset at various times during operation in the standardcalibration mode at various ambient conditions, and the differencesignal offset measurement being used to correct the difference signalfor component drift to provide a compensated difference measurementbetween the first and second values of the physical variable. 34.Apparatus as defined in claim 33 wherein the difference signalamplification means comprises differential inputs connected to ameasurement bridge comprising a first sensor having an impedanceresponsive to the physical variable, the first sensor being connected inseries to a first impedance, the first sensor and first impedance beingconnected across a measurement bridge potential for producing the firstsignal, and the first sensor and first impedance being connected inparallel to a second sensor having an impedance responsive to the samephysical variable as the first sensor, the second sensor being connectedin series with a second impedance, the series-connected second sensorand second impedance also being connected across the measurement bridgepotential for producing the second signal, and the inputs to thedifference signal amplification means being connected to the measurementbridge, such that each differential input is connected to themeasurement bridge at a different bridge node, situated between eitherthe first sensor and first impedance or between the second sensor andsecond impedance.
 35. Apparatus as defined in claim 34 wherein the firstand second sensors are configured to be subject to substantially thesame value of the physical variable, to which the sensors areresponsive, during the reference calibration mode, as well as during thestandard calibration mode, and wherein operation of the electronicdifference measurement apparatus in the reference calibration mode, withthe first and second sensors subject to substantially the same value ofthe physical variable, results in at least one physical variabledifference curve distinct from the difference reference curve, which isused during the standard calibration mode, with the first and secondsensors again subject to substantially the same value of the physicalvariable, to compare points on the physical variable difference curve,acquired at different times, so as to provide an offset measurementwhich substantially compensates for variations between the sensors, overthe range of ambient conditions in which the physical variabledifference curve is acquired, and additionally to compensate for timedrift in the variation over the ambient condition range in which thephysical variable difference curve was acquired during the referencecalibration mode.
 36. Apparatus as defined in claim 35 wherein themeasurement bridge potential is applied across a reference resistancebridge to generate at least one reference signal used to compensate theambient condition signal means for component time drift.
 37. Apparatusas defined in claim 36 wherein the potential across the referenceresistance bridge is at least one of a) applied to an input of theanalog to digital converter means in order to provide a ratiometriccompensation for variations in bridge voltage and b) generated using theanalog to digital converter means.
 38. Apparatus as defined in claim 35wherein at least one input to the ambient condition signal means isconnected to one of the first and second sensors of the measurementbridge.
 39. Apparatus as defined in claim 35 wherein the first andsecond sensors and the ambient condition signal means are responsive totemperature.
 40. Apparatus as defined in claim 39 wherein at least oneinput to the ambient condition signal means is connected to one of thefirst and second temperature sensors of the measurement bridge. 41.Apparatus as defined in claim 35, further comprising a timer and whereinat least one of the difference reference curve and physical variabledifference curve are re-acquired at a known time, relative to the timeat which the respective offset curves were last acquired, and comparedto previous versions of the respective offset curves to at least one ofa) estimate error in the linear translations, associated with thestandard calibration mode, and b) track trends in drift of therespective offset curves.
 42. Apparatus as defined in claim 41 whereinerrors associated with the linear translations in the standardcalibration mode are at least one of a) reported to a user and b) usedto limit the accuracy with which difference measurements are reported tothe user.
 43. Apparatus as defined in claim 35 wherein drift parametersassociated with at least one of the first sensor, second sensor, anddifference signal amplification means are stored in the memory means andused to determine at least one of the a) difference reference curve andb) physical variable difference curve.
 44. Apparatus as defined in claim34 wherein the measurement bridge potential is applied across areference resistance bridge to generate at least one reference signalused to compensate the ambient condition signal means for component timedrift. ambient condition signal means are responsive to temperature. 45.Apparatus as defined in claim 44 wherein the potential across thereference resistance bridge is at least one of a) applied to an input ofthe analog to digital converter means in order to provide a ratiometriccompensation for variations in bridge voltage and b) generated using theanalog to digital converter means.
 46. Apparatus as defined in claim 34wherein at least one input to the ambient condition signal means isconnected to one of the first and second sensors of the measurementbridge.
 47. Apparatus as defined in claim 34 wherein the first andsecond sensors and the ambient condition signal means are responsive totemperature.
 48. Apparatus as defined in claim 47 wherein at least oneinput to the ambient condition signal means is connected to one of thefirst and second temperature sensors of the measurement bridge. 49.Apparatus as defined in claim 34 wherein the potential across themeasurement bridge is at least one of a) applied to an input of theanalog to digital converter means in order to provide a ratiometriccompensation for variations in bridge voltage and b) generated using theanalog to digital converter means.
 50. Apparatus as defined in claim 33,further comprising a timer and wherein the difference reference curve isre-acquired at a known time, relative to the time at which thedifference reference curve was last acquired and compared to a previousversion of the difference reference curve to at least one of a) estimateerror in the linear translations, associated with the standardcalibration mode, and b) track trends in drift of the differencereference curve.
 51. Apparatus as defined in claim 50 wherein errorsassociated with the linear translations in the standard calibration modeare at least one of a) reported to a user and b) used to limit theaccuracy with which difference measurements are reported to the user.52. Apparatus as defined in claim 33 wherein at least one of the firstand second signals provides a substantially random signal source. 53.Apparatus as defined in claim 33 wherein the means for providing thefirst signal representative of a first value of the physical variablecomprises a first sensor and the means for providing the second signalrepresentative of a second value of the same physical variable comprisesa second sensor and wherein the first and second sensors are configuredto be subject to substantially the same value of the physical variable,to which the sensors are responsive, during the reference calibrationmode, as well as during the standard calibration mode, and whereinoperation of the electronic difference measurement apparatus in thereference calibration mode, with the first and second sensors subject tosubstantially the same value of the physical variable, results in atleast one physical variable difference curve distinct from thedifference reference curve, which is used during the standardcalibration mode, with the first and second sensors again subject tosubstantially the same value of the physical variable, to compare pointson the physical variable difference curve, acquired at different times,so as to provide an offset measurement which substantially compensatesfor variations between the sensors, over the range of ambient conditionsin which the physical variable difference curve is acquired, andadditionally to compensate for time drift in the variation over theambient condition range in which the physical variable difference curvewas acquired during the reference calibration mode.
 54. Apparatus asdefined in claim 53 wherein drift parameters associated with at leastone of the first sensor, second sensor, and difference signalamplification means are stored in the memory means and used to determineat least one of the a) difference reference curve and b) physicalvariable difference curve.
 55. Apparatus as defined in claim 33 whereinthe ambient condition signal means is responsive to temperature forproviding the third signal.
 56. Apparatus as defined in claim 33 whereina first ambient condition signal means is responsive to the ambientcondition of a first physical variable to provide the third signal andat least one second ambient condition signal means is responsive to asecond physical variable for providing a fourth signal, the fourthsignal being converted by the analog to digital converter means intodigital form and stored in the memory means for use by the computermeans for compensating the electronic difference measurement apparatusfor drift of electronic components due to variations in the ambientvalue of the second physical variable, and wherein at least two sets ofoffset curves are acquired over the range of the first physicalvariable, each set of offset curves being acquired at a different valueof ambient condition of the second physical variable within the range ofambient conditions of the second physical variable.
 57. Apparatus asdefined in claim 56 wherein at least one of the first and secondphysical variables is temperature.
 58. Apparatus as defined in claim 57wherein multiple second physical variables are representative of thesame physical parameter in different physical locations.
 59. Apparatusas defined in claim 56 wherein multiple second physical variables arerepresentative of the same physical parameter in different physicallocations.
 60. Apparatus as defined in claim 56 wherein drift parametersassociated with at least one of the first signal, second signal, anddifference signal amplification means are stored in the memory means andused to determine at least one of the a) difference reference curve andb) offset curves.
 61. Apparatus as defined in claim 33 wherein driftparameters associated with the difference signal amplification means arestored in the memory means and used to determine the differencereference curve.
 62. Apparatus as defined in claim 33, furthercomprising internal switches for connecting at least one of a) a signalrepresentative of ambient condition, b) a commnon signal, and c) atleast one reference signal to the inputs of the ambient condition signalmeans, the switches being operated under control of the computer means.63. Apparatus as defined in claim 62 wherein the switches are responsiveto a delayed action of a power switch and wherein the function of theinternal switches is performed by electronically controllable externalswitches operated under control of the computer means during operationin the reference calibration mode.
 64. Apparatus as defined in claim 33,further comprising internal switches for connecting at least one of a)the first signal, b) the second signal, c) a common signal, and d) atleast one reference signal to the inputs of the difference signalamplification means, the switches being operated under control of thecomputer means.
 65. Apparatus as defined in claim 64 wherein theswitches are responsive to a delayed action of a power switch andwherein the function of the internal switches is performed byelectronically controllable external switches operated under control ofthe computer means during operation in the reference calibration mode.66. Apparatus as defined in claim 33, further comprising: the apparatusbeing operable in a reference calibration mode, in which offset curvesare acquired over a range of ambient conditions and stored in the memorymeans, the offset curves comprising: a) at least one curverepresentative of offset for electronic components associated withambient condition measurements versus measured ambient condition, thecurve so generated being referred to as an ambient reference curve; andb) the difference reference curve; the apparatus additionally beingoperable in the standard calibration mode in which an ambient signaloffset and a difference signal offset that substantially compensate theambient reference curve and difference reference curve, respectively,for drift over time are determined at a current arbitrary ambientcondition by comparing measurements at the current arbitrary ambientcondition to previously stored values on the ambient reference curve anddifference reference curve and, for both the ambient reference curve anddifference reference curve, one of a) translating at least one of theambient reference curve and difference reference curve in order tosubstantially compensate the at least one reference curve for time driftassociated with ambient condition measurement and difference signalmeasurement, respectively, the respective reference curves aftertranslation being referred to as a translated ambient reference curveand a translated difference reference curve, respectively, b)determining at least one of an ambient signal offset and differencesignal offset by obtaining the difference between an estimated ambientreference measurement and an actual ambient reference measurement, andbetween an estimated difference reference measurement and an actualdifference reference measurement, respectively, the respective signaloffset being used to linearly translate ambient condition measurementsand difference measurements, respectively, in order to substantiallycompensate the respective measurements during the operational mode fordrift of components associated with ambient condition measurement anddifference measurement, respectively; and the apparatus for performingat least one difference measurement in the operational mode in which ameasurement representative of current ambient condition compensated fortime drift using at least one of the translated ambient reference curveand ambient signal offset determined during the standard calibrationmode provides an ambient condition measurement which is correlated to adifference signal offset measurement, the difference signal offsetmeasurement being at least one of a) determined from the translateddifference reference curve at the compensated ambient conditionmeasurement, b) equivalent to the difference signal offset determinedduring the standard calibration mode, and c) derived by empiricallycomparing values for the difference signal offset at various timesduring operation in the standard calibration mode, at various ambientconditions, and the difference signal offset measurement being used tocorrect the difference signal for component drift to provide acompensated difference measurement between the first and second valuesof the physical variable.
 67. Apparatus as defined in claim 66, furthercomprising a timer and wherein the ambient reference curve anddifference reference curve are re-acquired at a known time, relative tothe time at which the respective offset curves were last acquired andcompared to previous versions of the respective offset curves to atleast one of a) estimate error in the linear translations, associatedwith the standard calibration mode, and b) track trends in drift of therespective offset curves.
 68. Apparatus as defined in claim 66 whereinthe means for providing the first signal representative of a first valueof the physical variable comprises a first sensor and the means forproviding the second signal representative of a second value of the samephysical variable comprises a second sensor and wherein drift parametersassociated with at least one of the first sensor, second sensor,difference signal amplification means, and ambient condition signalmeans are stored in the memory means and used to determine at least oneof a) the ambient reference curve, and b) the difference referencecurve.